Active matrix substrate and drive circuit thereof

ABSTRACT

An active matrix substrate used in a display device or the like capable of making substantially uniform the level shift generated in the pixel potential caused by the distribution of resistance and capacity in each signal line is disclosed. On the TFT substrate which is an active matrix substrate including a common electrode line formed parallel to the scan signal line, in order to eliminate non-uniformity of the level shift of the pixel potential generated at the scan signal fall, each pixel circuit is formed so that the capacity between the scan signal line and the pixel electrode becomes greater as electrically going farther from the scan signal line drive circuit and going farther from the common electrode line drive circuit. Embodiments can be applied especially to an active matrix substrate used in a liquid crystal display device, an EL display device, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority under 35 U.S.C.§§120/121 to U.S. patent application Ser. No. 11/631,652, filed on Jan.5, 2007 now U.S. Pat. No. 8,264,434, which is a National Stage ofInternational Application No. PCT/JP05/11632, filed on Jun. 24, 2005,and which claims the benefit of Japanese Patent Application No.2004-207738, filed on Jul. 14, 2004. The disclosures of each of theabove applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to active matrix substrates and drivecircuits for the substrates used in matrix type liquid crystal displaydevices, EL (Electroluminescence) display devices and so on. Morespecifically, the present invention relates to an active matrixsubstrate formed with a plurality of data signal lines and a pluralityof scanning lines to cross with each other in a grid pattern and amatrix of pixel circuits each including a switching element provided bya field-effect transistor such as a thin-film transistor and a voltageholding capacitor. The invention also relates to a drive circuit for thesubstrate.

BACKGROUND ART

Active matrix substrates are used widely in active matrix type displaydevices such as liquid crystal display devices and EL display devices,as well as in a variety of active matrix type sensors and otherproducts. In particular, special attentions are being paid to liquidcrystal display devices in which each display pixel is provided with aswitching element such as a thin-film transistor (hereinafterabbreviated as TFT) which is a type of field-effect transistors, sincethese display devices are capable of displaying excellent images withoutcrosstalk between mutually adjacent display pixels even if there is alarge number of display pixels.

Such an active matrix type liquid crystal display device primarilyincludes a liquid crystal display panel and a drive circuit for thepanel. The liquid crystal display panel includes a pair of electrodesubstrates sandwiching a liquid crystal layer. Each electrode substratehas its outer surface covered by a polarizer.

Of the pair of electrode substrates, one is an active matrix substratecalled TFT substrate. The TFT substrate includes an insulating substratemade of glass for example, formed with a plurality of data signal linesand a plurality of scanning signal lines crossing with each other in agrid pattern and in addition, a plurality of common electrode lines areformed to extend in parallel to the scanning signal lines. Further,correspondingly to each of the intersections made by the data signallines and the scanning signal lines, a matrix of pixel circuits areformed. Each pixel circuit includes a pixel electrode for acorresponding pixel that serves to form images to be displayed, pixelcapacities formed by the pixel electrode and other elements such as anopposed electrode which will be described later, and a TFT which servesas a switching element. The other electrode substrate, called opposedsubstrate, is provided by a transparent insulating substrate made ofglass for example, having its entire surface laminated with a layer ofopposed electrode and then with an alignment film.

The active matrix type liquid crystal display device includes, as adrive circuit for the liquid crystal display panel configured as theabove, a scanning signal line drive circuit connected with the scanningsignal lines, a data signal line drive circuit connected with the datasignal lines, a common electrode line drive circuit connected with thecommon electrode lines, and an opposed electrode drive circuit connectedwith the opposed electrode.

The data signal line drive circuit generates, based on image signalsreceived from an outside signal-source for example, a plurality of datasignals successively in the form of analog voltage representing pixelvalues in each horizontal scanning line of the image to be displayed inthe liquid crystal display panel, and applies these data signalsrespectively to the data signal lines in the liquid crystal displaypanel. The scanning signal line drive circuit selects the scanningsignal lines in the liquid crystal display panel sequentially for eachhorizontal scanning period, and applies an active scanning signal (avoltage for turning ON the TFTs in the pixel circuit) to the selectedscanning signal line, in each frame period (each vertical scanningperiod) for displaying an image on the liquid crystal display panel. Thecommon electrode line drive circuit and the opposed electrode drivecircuit apply signals to the common electrode lines and the opposedelectrode respectively; these signals give electric potentials thatserve as baseline voltages for voltages to be applied to the liquidcrystal layer of the liquid crystal display panel.

As described above, the data signal lines are supplied with respectivedata signals, the scanning signal lines are supplied with respectivescanning signals, whereby the pixel electrode in each pixel circuit ofthe liquid crystal display panel is supplied with a voltage representingthe value of the pixel for the image to be displayed, with the potentialat the opposed electrode serving as the baseline voltage, and thesupplied voltage is held at the pixel capacity in each pixel circuit.Thus, a voltage which equals to the potential difference between eachpixel electrode and the opposed electrode is applied to the liquidcrystal layer. By controlling optical transmittance based on the appliedvoltage, the liquid crystal display panel displays an image representedby the image signals received from e.g. an outside signal-source.

FIG. 19 is a circuit diagram which shows a configuration of a pixelcircuit in a TFT substrate serving as an active matrix substrate used ina liquid crystal display device as described above. A pixel circuit P(i,j) corresponds to one of the intersections made by the data signal linesand the scanning signal lines, and includes: a TFT 102 which has asource electrode connected with a data signal line S(i) passing thecorresponding intersection, and a gate electrode connected with ascanning signal line G(j) passing the same intersection; and a pixelelectrode 103 connected with a drain electrode of the TFT 102. The pixelelectrode 103 and the opposed electrode form a liquid-crystal capacityClc. The pixel electrode 103 and a common electrode line CS(j) providedalong the scanning signal line G(j) form a common-electrode capacity(may also called “supplemental capacity”) Ccs, and the pixel electrode103 and the scanning signal line G(j) form a parasitic capacity Cgd.

Hereinafter, reference will be made to FIG. 4-(A) through FIG. 4-(D),FIG. 9 and FIG. 19, to describe a conventional method of driving theabove-described TFT substrate in a liquid crystal display device. As amatter of well known fact, liquid crystal displays need AC driving inorder to reduce burning images on the screen and display deterioration.The following description of a conventional driving method will assumethat a frame-inversion driving method which is a type of AC driving isused.

FIG. 4-(A) through 4-(D) are voltage waveform charts of various voltagesignals Vg(j), Vs(i), Vcs, Vcom in the TFT substrate and a waveform of apotential of the pixel electrode (hereinafter may also called “pixelpotential”) Vd (i, j) in two consecutive frame periods, i.e. a firstframe period TF1 and a second frame period TF2. As shown in FIG. 4-(A),in the first frame period TF1, a voltage serving as a scanning signal(hereinafter called “scanning voltage”) Vgh is applied from the scanningsignal line drive circuit to the gate electrode g (i, j) of the TFT 102in a pixel circuit P(i, j). This turns ON the TFT 102 (into a conductivestate), where a voltage serving as a data signal (hereinafter called“data signal voltage”) Vsp applied from the data signal line drivecircuit to the data signal line S(i) is supplied to the pixel electrode103 via the source electrode and the drain electrode of the TFT 102.Thus, the data signal voltage Vsp becomes a positive-polarity voltagewith respect to the opposed-electrode potential Vcom (=common electrodepotential Vcs), and is written to a pixel capacity Cpix which is formedby the pixel electrode 103 and other electrodes. As shown in FIG. 4-(D),the pixel electrode 103 holds the pixel potential Vdp until a scanningvoltage Vgh is applied in the next frame period, i.e. the second frameperiod TF2. As shown in FIG. 19, the pixel capacity Cpix for holding thepixel potential Vdp is made of the liquid-crystal capacity Clc, thecommon-electrode capacity Ccs and the parasitic capacity Cgd. Meanwhile,the opposed electrode is set to a predetermined opposed-electrodepotential Vcom by the opposed electrode drive circuit. Therefore, theliquid crystal sandwiched between the pixel electrode and the opposedelectrode makes a response in accordance with the potential differencebetween the pixel potential Vdp and the opposed-electrode potentialVcom, achieving a display of the image.

Likewise, as shown in FIG. 4-(A), in the second frame period TF2, uponapplication of the scanning voltage Vgh from the scanning signal linedrive circuit to the gate electrode g(i, j) of the TFT 102 in the pixelcircuit P(i, j), the TFT 102 is turned ON, where a data signal voltageVsn which is applied from the data signal line drive circuit to the datasignal line S(i) is supplied to the pixel electrode 103 via the sourceelectrode and the drain electrode of the TFT 102. Thus, the data signalvoltage Vsn becomes a negative-polarity voltage with respect to theopposed-electrode potential Vcom (=Vcs), and is written into the pixelcapacity Cpix. The pixel electrode 103 holds the pixel potential Vdnuntil the scanning voltage Vgh is applied in the next frame period.Thus, the liquid crystal sandwiched between the pixel electrode and theopposed electrode makes a response in accordance with the potentialdifference between the pixel potential Vdn and the opposed-electrodepotential Vcom, achieving a display of the image, in an AC driving of aliquid crystal.

As shown in FIG. 19, a parasitic capacity Cgd is unavoidably formedbetween the scanning signal line G(j) and the pixel electrode 103 ineach pixel circuit P(i, j) as a nature of the configuration. Therefore,as shown in FIG. 4-(D), at the time when the active scanning signalvoltage, i.e. the scanning voltage Vgh falls down to the non activescanning signal voltage, i.e. the scanning voltage Vgl (represented by atime point ta in the figure), a level shift ΔVd occurs in the pixelpotential Vd due to the parasitic capacity Cgd. It should be noted herethat in FIG. 4-(D), the level shift of the pixel potential Vd (i, j) inthe pixel circuit P(i, j) in the first frame period (in the period whena positive voltage is applied to the liquid crystal layer) TF1 (or moreaccurately, a level shift at a time point tb which is a time point wellafter the time point ta) is indicated by a symbol “ΔVdp (i, j)” whereasthe level shift of the pixel potential Vd (i, j) in the pixel circuitP(i, j) in the second frame period (the period when a negative voltageis applied to the liquid crystal layer) TF2 is indicated by a symbol“ΔVdn (i, j)”. However, when there is no need to specifically clarifythe pixel circuit or the frame period, these level shifts will beindicated by a common symbol “ΔVd” as used in the above (The same willapply hereinafter).

The level shift ΔVd which occurs in the pixel potential Vd due to theparasitic capacity Cgd which is formed unavoidably in the TFT 102 isexpressed as follows:ΔVd=Vgpp·Cgd/CpixVgpp=Vgl−VghCpix=C1c+Ccs+CgdThe level shift causes such problems as flickers in the displayed imageand decreased quality of the displayed image. For this reason,occurrence of the level shift ΔVd as the above is not preferable forliquid crystal display devices which are supposed to achieve ever higherfineness and quality.

Meanwhile, there has been a number of methods (means) proposed foreliminating or reducing the level shift ΔVd as described above. Forexample, a method has been proposed in which a bias is given to thepotential at the opposed electrode so that the level shift ΔVd caused bythe parasitic capacity Cgd will be reduced in advance. Also, JP-A Hei11-281957 Gazette (This corresponds to U.S. Pat. No. 6,359,607, thecontents of which is incorporated herein by reference), discloses amethod in which the level shift variation in the pixel potential isreduced by controlling the fall of the scanning signal. Further, JP-A2001-33758 Gazette discloses a method in which the level shift variationin the pixel potential (electric potential of the pixel electrode) byconnecting a plurality of variable power sources to the common electrodeline.

-   [Patent Document 1] JP-A 2002-202493 Gazette-   [Patent Document 2] JP-A 2001-33758 Gazette-   [Patent Document 3] JP-A Hei 11-281957 Gazette-   [Patent Document 4] JP-A Hei 11-84428 Gazette-   [Patent Document 5] JP-A Hei 10-39328 Gazette-   [Patent Document 6] JP-A Hei 5-232512 Gazette

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the manufacture of the TFT substrate as an active matrixsubstrate for use in liquid crystal display devices, it is difficult toform ideal signal lines which are free from signal propagation delay, ona transparent insulating substrate provided by glass for example. Acertain degree of signal propagation delay is unavoidable.

For example, scanning signal lines formed on the TFT substrate must betreated as a distributed constant wire which has a wiring resistance, awiring capacity, etc. This means that the signal lines have a signalpropagation delay characteristic. Therefore, the voltage waveform of thescanning signal Vg(j) in a scanning signal line deforms with increasingdistance from the point where the scanning signal Vg(j) is applied bythe scanning signal line drive circuit (i.e. from the input end of thescanning signal Vg(j)). Hence, the absolute value |ΔVd| of the levelshift ΔVd caused in the pixel potential Vd by the parasitic capacity Cgddecreases with increasing distance from the input end of the scanningsignal Vg(j) in the scanning signal line.

As described, the level shift ΔVd has different values depending on thelocation of pixel circuit, and therefore not uniform in the screen (inthe TFT substrate). Therefore, if the method of giving a bias to thepotential Vcom of the opposed electrode in an attempt that the levelshift ΔVd of the pixel potential Vd will be reduced in advance, it isnot possible to achieve sufficient elimination of the flickers in thedisplayed image and display quality deterioration which are caused bythe level shift ΔVd, by simply applying a uniform bias to the opposedelectrode. Specifically, with increase in the screen size and in thelevel of fineness, non-uniformity of the level shift ΔVd grows to alevel beyond the viability of the above-described method, becomingunable to provide sufficient AC driving to each liquid crystal blockwhich corresponds to a pixel, leading to problems such as flickers inthe displayed image, burning images on the screen due to DC componentsapplied to the liquid crystal, and so on.

To this problem, Patent Document 1 (Japanese Patent Laid-Open No.2002-202493 Gazette) discloses a liquid crystal display device in whichpower supply for the opposed electrode that faces the pixel electrode isprovided at least at two locations, i.e. on the input end side and onthe terminating end side of the scanning signal line, and opposedvoltages are supplied to the above-mentioned at least two power supplysources so that the potential of the opposed electrode will increasefrom the input-end side toward the output-end side. However, such aconfiguration complicates the structure for driving the opposedelectrode, and furthermore, results in increased power consumption dueto the current flow between the two power supply sources for the opposedelectrode.

Patent Document 2 (Japanese Patent Laid-Open No. 2001-33758 Gazette)discloses another method: Specifically, level shift variation in thepixel potential is reduced by connecting a plurality of variable powersources to the common electrode line. If this method is used, it ispossible to relatively cancel the level shift, with potential variationof the electrode which is opposed to the pixel electrode. However, themethod requires a plurality of variable power sources in order to drivethe common electrode.

Further, Patent Document 3 (Japanese Patent Laid-Open No. Hei 11-281957Gazette) disclose another method: Specifically, level shift variation ofthe pixel potential is reduced by controlling the fall of the scanningsignal. If this method is to be used, a special drive circuit must beprovided. Further, time for charging the pixel capacity must be reduced.

Patent Document 4 (Japanese Patent Laid-Open Hei 11-84428 Gazette (Thiscorresponds to U.S. Pat. No. 6,249,325 and No. 6,504,585, the contentsof which are incorporated herein by reference)) discloses a liquidcrystal display device: Toward the goal of uniformalizing the levelshift of the pixel potential, an arrangement is made so that thecapacity between the gate electrode and the source electrode of thethin-film transistor (TFT) formed in the liquid crystal display panelwill be smaller on the input side of the gate signal line, and greateron the side of terminating end. However, due to lack of consideration tovariation in the amount of charge in e.g. the pixel capacity caused bythe current flows in the TFT from the time when the gate signal startsto fall to the time when the signal completes the fall (details will bedescribed later), it is not possible to achieve sufficient eliminationof the level shift non-uniformity of the pixel potential by thedisclosed arrangement alone.

As will be described later, the inventor of the present invention foundthat in order to eliminate the level shift non-uniformity of the pixelpotential, consideration must be made to influences of the parasiticcapacity between the scanning signal line and the common electrode lineand influences of the signal propagation delay characteristic in thecommon electrode line. However, these influences are not considered inany of the conventional techniques including the technique disclosed inPatent Document 4 (Japanese Patent Laid-Open Hei 11-84428 Gazette), sothe level shift non-uniformity of the pixel potential cannot beeliminated sufficiently for this reason, either. Further, PatentDocument 5 (Japanese Patent Laid-Open Hei 10-39328 Gazette (Thiscorresponds to U.S. Pat. No. 6,028,650, the content of which isincorporated herein by reference)) discloses a liquid crystal displaydevice, in which each of the pixel electrodes is provided with asupplemental capacity arranged in such a way that its capacity valuewill decrease as the distance increases from the input end of the gatesignal line connected with the pixel electrode. However, again, such anarrangement as the above cannot eliminate the level shift non-uniformityof the pixel potential, for the same reason.

It is therefore a first object of the present invention to provide anactive matrix substrate in which the level shift caused in the pixelpotential by the resistance and capacity distribution in each signalline is substantially uniformalized within the substrate. Further, asecond object of the present invention is to provide a drive circuitwhich drives an active matrix substrate in such a way that the levelshifts caused in the pixel potential by the resistance and capacitydistribution in each signal line are substantially uniformalized withinthe substrate. Further, a third object of the present invention is toprovide a display device capable of giving a high quality display ofimages by substantially uniformalizing the level shift in the pixelpotential within the active matrix substrate thereby canceling displaynon-uniformity.

Means for Solving the Problems

A first aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines; and

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; and a gate electrode connected with one of thescanning signal lines that passes through the correspondingintersection; the field-effect transistor assuming a conductive stateupon application of a predetermined ON voltage while assuming anonconductive state upon application of a predetermined OFF voltage, tothe gate electrode based on the source electrode as a baseline; and

a voltage holding electrode connected with a drain electrode of thefield-effect transistor, providing a predetermined voltage holdingcapacitor.

Each pixel circuit is formed so that a value given by an equation belowwill be substantially equal among the pixel circuits:(Vgpp·Cgd+ΔQd)/Cpixwhere Vgpp represents an amount of potential change at the gateelectrode from a time when a gate signal, which is given to the gateelectrode of the field-effect transistor via the scanning signal line,starts its transition from the ON voltage to the OFF voltage to a timewhen the transition is complete; Cgd represents an electrostaticcapacity between the gate electrode and the drain electrode in thefield-effect transistor; ΔQd represents an amount of charge which movesthrough the field-effect transistor to the voltage holding electrodefrom the time when the gate signal starts its transition from the ONvoltage to the OFF voltage to the time when the transition is complete;and Cpix represents a sum of electrostatic capacities formed between thedrain electrode of the field-effect transistor or the voltage holdingelectrode and other electrodes in each pixel circuit.

In the above-described configuration, an arrangement in order that thevalue given by the above-described equation will be substantially equalamong the pixel circuits may be that only one setting on a parametersuch as a characteristic of the TFT or one of the electrostaticcapacities (various electrostatic capacities formed between the pixelelectrode and other electrodes) is changed, or may be that changes aremade on a selected combination of these parameters.

A second aspect of the present invention provides the active matrixsubstrate according to the first aspect of the present invention whichfurther includes

a common electrode line disposed for formation of predeterminedelectrostatic capacities between itself and the voltage holdingelectrodes.

The charge amount ΔQd is determined, taking into account a parasiticcapacity between the scanning signal line and the common electrode lineand/or a signal propagation delay characteristic of the common electrodeline.

It should be appreciated that often, the common electrode line isdisposed in parallel to the scanning signal line; however, the presentinvention is not limited to this as long as a predeterminedelectrostatic capacity (an equivalent of the common-electrode capacityor the supplemental capacity) is formed between the line and the pixelelectrode. Further, the common electrode line may ride on a plurality ofscanning signal lines, or a plurality of data signal lines. A pluralityof the common electrode lines may be provided per a pixel circuit or pera pixel electrode, or the common electrode line may be provided as aplate. The freedom in the configuration of common electrode lines as theabove applies to all of the aspects to be described here below.

A third aspect of the present invention provides the active matrixsubstrate according to the first aspect of the present invention,wherein the electrostatic capacity Cgd in each pixel circuit is formedso that the value given by the equation (Vgpp·Cgd+ΔQd)/Cpix issubstantially equal among the pixel circuits.

A fourth aspect of the present invention provides the active matrixsubstrate according the first aspect of the present invention, whereinone of the electrostatic capacities which are formed between the drainelectrode of the field-effect transistor or the voltage holdingelectrode and other electrodes other than the electrostatic capacity Cgdbetween the gate electrode and the drain electrode of the field-effecttransistor is formed in each pixel circuit so that the value given bythe equation (Vgpp·Cgd+ΔQd)/Cpix is substantially equal among the pixelcircuits.

A fifth aspect of the present invention provides the active matrixsubstrate according to the first aspect of the present invention,wherein the field-effect transistor in each pixel circuit has a channellength and a channel width so selected that the value given by theequation (Vgpp·Cgd+ΔQd)/Cpix is substantially equal among the pixelcircuits.

A sixth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines;

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; and a gate electrode connected with one of thescanning signal lines that passes through the correspondingintersection; and a voltage holding electrode connected with a drainelectrode of the field-effect transistor, providing a predeterminedvoltage holding capacitor.

An electrostatic capacity Cgd between the gate electrode and the drainelectrode in the field-effect transistor increases whereas a rate of theincrease of the electrostatic capacity Cgd decreases with an increasingelectrical distance from a location of signal application for drivingthe scanning signal line which passes through the correspondingintersection.

A seventh aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines; and

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and avoltage holding electrode connected with a drain electrode of thefield-effect transistor, providing a predetermined voltage holdingcapacitor.

An area of overlap between the electrode constituting the scanningsignal line that passes through the corresponding intersection and thevoltage holding electrode or the drain electrode of the field-effecttransistor increases whereas a rate of the increase of the areadecreases with an increasing electrical distance from a location ofsignal application for driving the scanning signal line which passesthrough the corresponding intersection.

An eighth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines; and

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and avoltage holding electrode connected with a drain electrode of thefield-effect transistor, providing a predetermined voltage holdingcapacitor.

A ratio L/W between a channel length L and a channel width W in thefield-effect transistor increases whereas a rate of the increase in theratio L/W decreases with an increasing electrical distance from alocation of signal application for driving the scanning signal linewhich passes through the corresponding intersection.

A ninth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines; and

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines;

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and avoltage holding electrode connected with a drain electrode of thefield-effect transistor, providing a predetermined voltage holdingcapacitor.

At least one of the electrostatic capacities which are formed betweenthe drain electrode of the field-effect transistor or the voltageholding electrode and other electrodes other than the electrostaticcapacity Cgd between the gate electrode and the drain electrode of thefield-effect transistor decreases while a rate of the decrease of saidat least one electrostatic capacity decreases with an increasingelectrical distance from a location of signal application for drivingthe scanning signal line which passes through the correspondingintersection.

A tenth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines;

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines; and

a common electrode line disposed for formation of a predeterminedelectric capacity in each pixel circuit;

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and avoltage holding electrode connected with a drain electrode of thefield-effect transistor, providing the predetermined electrostaticcapacity between itself and the common electrode line.

An electrostatic capacity Cgd between the gate electrode and the drainelectrode in the field-effect transistor increases with an increasingelectrical distance from a location where an electric potential to besupplied to the common electrode line is applied to the common electrodeline.

An eleventh aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines;

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines; and

a common electrode line disposed to form a predetermined electriccapacity in each pixel circuit.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and

a voltage holding electrode connected with a drain electrode of thefield-effect transistor, providing the predetermined electrostaticcapacity between itself and the common electrode line.

The pixel circuits include a first, a second and a third pixel circuits,the first pixel circuit is closer to an end of the common electrode lineand farther from a center of the common electrode line than the secondpixel circuit. The third pixel circuit is closer to another end of thecommon electrode line and farther from the center of the commonelectrode line than the second pixel circuit, and

an electrostatic capacity Cgd between the gate electrode and the drainelectrode in the field-effect transistor in the second pixel circuit isgreater than an electrostatic capacity Cgd between the gate electrodeand the drain electrode in the field-effect transistor in both of thefirst and the third pixel circuits.

A twelfth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines;

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines; and

a common electrode line disposed to form a predetermined electriccapacity in each pixel circuit.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and avoltage holding electrode connected with a drain electrode of thefield-effect transistor, providing the predetermined electrostaticcapacity between itself and the common electrode line.

An area of overlap between the electrode constituting the scanningsignal line that passes through the corresponding intersection and thevoltage holding electrode or the drain electrode of the field-effecttransistor increases with an increasing electrical distance from alocation where an electric potential to be supplied to the commonelectrode line is applied to the common electrode line.

A thirteenth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines;

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines; and

a common electrode line disposed to form a predetermined electriccapacity in each pixel circuit.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection.

A voltage holding electrode connected with a drain electrode of thefield-effect transistor, providing the predetermined electrostaticcapacity between itself and the common electrode line.

The pixel circuits include a first, a second and a third pixel circuits,the first pixel circuit is closer to an end of the common electrode lineand farther from a center of the common electrode line than the secondpixel circuit. The third pixel circuit is closer to another end of thecommon electrode line and farther from the center of the commonelectrode line than the second pixel circuit, and

an area of overlap between the electrode constituting the scanningsignal line that passes through the corresponding intersection and thevoltage holding electrode or the drain electrode of the field-effecttransistor in the second pixel circuit is greater

than an area of overlap between the electrode constituting the scanningsignal line that passes through the corresponding intersection and thevoltage holding electrode or the drain electrode of the field-effecttransistor in the first pixel circuit, and greater

than an area of overlap between the electrode constituting the scanningsignal line that passes through the corresponding intersection and thevoltage holding electrode or the drain electrode of the field-effecttransistor in the third pixel circuit.

A fourteenth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines;

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines; and

a common electrode line disposed to form a predetermined electriccapacity in each pixel circuit.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and avoltage holding electrode connected with a drain electrode of thefield-effect transistor, providing the predetermined electrostaticcapacity between itself and the common electrode line.

A ratio L/W between a channel length L and a channel width W in thefield-effect transistor increases with an increasing electrical distancefrom a location where an electric potential to be supplied to the commonelectrode line is applied to the common electrode line.

A fifteenth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines;

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines; and

a common electrode line disposed to form a predetermined electriccapacity in each pixel circuit.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and

a voltage holding electrode connected with a drain electrode of thefield-effect transistor, providing the predetermined electrostaticcapacity between itself and the common electrode line.

The pixel circuits include a first, a second and a third pixel circuits,the first pixel circuit is closer to an end of the common electrode lineand farther from a center of the common electrode line than the secondpixel circuit. The third pixel circuit is closer to another end of thecommon electrode line and farther from the center of the commonelectrode line than the second pixel circuit, and

a ratio L/W between a channel length L and a channel width W in thefield-effect transistor in the second pixel circuit is greater than aratio L/W between the channel length L and the channel width W in thefield-effect transistor in both of the first and the third pixelcircuits.

A sixteenth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines;

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines; and

a common electrode line disposed to form a predetermined electriccapacity in each pixel circuit.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and avoltage holding electrode connected with a drain electrode of thefield-effect transistor, providing the predetermined electrostaticcapacity between itself and the common electrode line.

One of the electrostatic capacities which are formed between the drainelectrode of the field-effect transistor or the voltage holdingelectrode and other electrodes other than an electrostatic capacity Cgdbetween the gate electrode and the drain electrode of the field-effecttransistor decreases with an increasing electrical distance from alocation where an electric potential to be supplied to the commonelectrode line is applied to the common electrode line.

A seventeenth aspect of the present invention provides an active matrixsubstrate which includes:

data signal lines each for one of data signals;

scanning signal lines crossing with the data signal lines;

a matrix of pixel circuits each corresponding to one of intersectionsmade by the data signal lines and the scanning signal lines; and

a common electrode line disposed to form a predetermined electriccapacity in each pixel circuit.

Each pixel circuit includes:

a field-effect transistor having: a source electrode connected with oneof the data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and

a voltage holding electrode connected with a drain electrode of thefield-effect transistor, providing the predetermined electrostaticcapacity between itself and the common electrode line.

The pixel circuits include a first, a second and a third pixel circuits,the first pixel circuit is closer to an end of the common electrode lineand farther from a center of the common electrode line than the secondpixel circuit. The third pixel circuit is closer to another end of thecommon electrode line and farther from the center of the commonelectrode line than the second pixel circuit, and

one of the electrostatic capacities which are formed between the drainelectrode of the field-effect transistor or the voltage holdingelectrode and other electrodes in the second circuit other than anelectrostatic capacity Cgd between the gate electrode and the drainelectrode of the field-effect transistor in the second circuit issmaller than one of the electrostatic capacities which are formedbetween the drain electrode of the field-effect transistor or thevoltage holding electrode and other electrodes in the first circuitother than the electrostatic capacity Cgd between the gate electrode andthe drain electrode of the field-effect transistor in the first circuit,and smaller than one of electrostatic capacities which are formedbetween the drain electrode of the field-effect transistor or thevoltage holding electrode and other electrodes in the third circuitother than the electrostatic capacity Cgd between the gate electrode andthe drain electrode of the field-effect transistor in the third circuit.

An eighteenth aspect of the present invention provides a drive circuitfor the active matrix substrate according to the first through fifteenthaspect of the present invention. The drive circuit includes:

a scanning signal line drive circuit for selectively driving thescanning signal line by applying predetermined scanning signalsrespectively to the scanning signal lines.

The scanning signal line drive circuit controls a speed of electricpotential change when the scanning signals make a transition from apredetermined ON voltage which turns the field-effect transistors into aconductive state to a predetermined OFF voltage which turns thefield-effect transistors into a nonconductive state.

A nineteenth aspect of the present invention provides the drive circuitaccording to the eighteenth aspect of the present invention. Thescanning signal line drive circuit controls the speed of electricpotential change of the scanning signals to be outputted from thescanning signal line drive circuit, based on a signal propagation delaycharacteristic of the scanning signal lines, so that the speed ofelectric potential change will be substantially equal regardless of thelocation on the scanning signal lines.

A twentieth aspect of the present invention provides a display devicewhich includes the active matrix substrate according to one of the firstthrough fifteenth aspects of the present invention, and a drive circuitfor driving the active matrix substrate.

A twenty-first aspect of the present invention provides a display deviceaccording to the twentieth aspect of the present invention.

The drive circuit includes a scanning signal line drive circuit forselectively driving the scanning signal line by applying predeterminedscanning signals respectively to the scanning signal lines.

The scanning signal line drive circuit controls a speed of electricpotential change when the scanning signals make a transition from apredetermined ON voltage which turns the field-effect transistors into aconductive state to a predetermined OFF voltage which turns thefield-effect transistors into a nonconductive state.

A twenty-second aspect of the present invention provides a displaydevice according to the twenty-first aspect of the present invention.

The scanning signal line drive circuit controls a speed of electricpotential change of the scanning signals to be outputted from thescanning signal line drive circuit based on a signal propagation delaycharacteristic of the scanning signal line so that the speed of electricpotential change will be substantially equal regardless of the locationon the scanning signal lines.

Advantages of the Invention

According to the first aspect of the present invention, each pixelcircuit is formed so that a value given by the equation(Vgpp·Cgd+ΔQd)/Cpix will be substantially equal among the pixel circuitsin the active matrix substrate toward the goal of eliminating electricpotential variations among the voltage holding electrodes, i.e. levelshift differences among pixel potentials, caused by difference in theamount of charge which moves through the TFT to the voltage holdingelectrode (representing the pixel electrode in the crystal displaydevice) due to difference in delay in various signal lines in each pixelcircuit. This enables to substantially uniformalize the level shift ΔVdin each pixel circuit.

According to the second aspect of the present invention, the chargeamount ΔQd is determined, taking into account a parasitic capacitybetween the scanning signal line and the common electrode line and/or asignal propagation delay characteristic of the common electrode line.Therefore, level shift non-uniformity of the pixel potential iseliminated or reduced sufficiently in an active matrix substrate whichis formed with common electrode lines.

According to the third aspect of the present invention, theelectrostatic capacity Cgd is formed so that a value given by theequation (Vgpp·Cgd+ΔQd)/Cpix will be substantially equal among the pixelcircuits, whereby the same advantages as offered by the first aspect ofthe present invention can be enjoyed. According to the fourth aspect ofthe present invention, one of the electrostatic capacities which areformed between the drain electrode of the field-effect transistor or thevoltage holding electrode and other electrodes other than theelectrostatic capacity Cgd between the gate electrode and the drainelectrode of the field-effect transistor is formed in each pixel circuitso that the value given by the equation (Vgpp·Cgd+ΔQd)/Cpix issubstantially equal among the pixel circuits, whereby the sameadvantages as offered by the first aspect of the present invention canbe enjoyed. According to the fifth aspect of the present invention, thefield-effect transistor in each pixel circuit has a channel length and achannel width so selected that the value given by the equation(Vgpp·Cgd+ΔQd)/Cpix is substantially equal among the pixel circuits,whereby the same advantages as offered by the first aspect of thepresent invention can be enjoyed.

In both of the sixth and the seventh aspects of the present invention,each pixel circuit is formed so that the electrostatic capacity Cgdincreases whereas a rate of the increase decreases with an increasingelectrical distance from an inputting end of the scanning signal line(where the scanning signal is applied). This reduces nonuniformity ofthe level shift in the pixel potential caused by difference in theamount of charge which moves to the voltage holding electrode due to thesignal propagation delay characteristic of the scanning signal lines,and uniformalizes the level shift distribution.

According to the eighth aspect of the present invention, each pixelcircuit is formed so that a ratio L/W between a channel length L and achannel width W in the field-effect transistor increases whereas a rateof the increase decreases with an increasing electrical distance from aninputting end of the scanning signal line. This reduces nonuniformity ofthe level shift in the pixel potential caused by difference in theamount of charge which moves to the voltage holding electrode due to thesignal propagation delay characteristic of the scanning signal lines,and uniformalizes the level shift distribution.

According to the ninth aspect of the present invention, each pixelcircuit is formed so that at least one of the electrostatic capacitieswhich are formed between the drain electrode of the field-effecttransistor or the voltage holding electrode and other electrodes otherthan the electrostatic capacity Cgd between the gate electrode and thedrain electrode of the field-effect transistor decreases while a rate ofthe decrease of said at least one electrostatic capacity decreases withan increasing electrical distance from a location of signal applicationto the scanning signal line. This reduces nonuniformity of the levelshift in the pixel potential caused by difference in the amount ofcharge which moves to the voltage holding electrode due to the signalpropagation delay characteristic of the scanning signal lines, anduniformalizes the level shift distribution.

In both of the tenth and the twelfth aspects of the present invention,each pixel circuit is formed so that the electrostatic capacity Cgdincreases with an increasing electrical distance from an inputting endof the common electrode line (where a common electrode potential isapplied). This reduces nonuniformity of the level shift in the pixelpotential caused by difference in the amount of charge which moves tothe voltage holding electrode due to parasitic capacities between thescanning signal line and the common electrode line as well as the signalpropagation delay characteristic in the common electrode lines, andtherefore uniformalizes the level shift distribution.

Both the eleventh and the thirteenth aspects of the present inventioncover a case in which an active matrix substrate is formed with commonelectrode lines and a common electrode potential (common electrodesignal) is applied from both ends of each common electrode line. Withthe above, each pixel circuit is formed so that the electrostaticcapacity Cgd increases with an increasing electrical distance frominputting ends of the common electrode line (where a common electrodepotential is applied). This reduces nonuniformity of the level shift inthe pixel potential caused by difference in the amount of charge whichmoves to the voltage holding electrode due to parasitic capacitiesbetween the scanning signal line and the common electrode line as wellas the signal propagation delay characteristic in the common electrodelines, and therefore uniformalizes the level shift distribution.

According to the fourteenth aspect of the present invention, each pixelcircuit is formed so that a ratio L/W between a channel length L and achannel width W in the field-effect transistor increases with anincreasing electrical distance from an inputting end of the commonelectrode line. This reduces nonuniformity of the level shift in thepixel potential caused by difference in the amount of charge which movesto the voltage holding electrode due to parasitic capacities between thescanning signal line and the common electrode line as well as the signalpropagation delay characteristic in the common electrode lines, andtherefore uniformalizes the level shift distribution.

The fifteenth aspect of the present invention covers a case in which anactive matrix substrate is formed with common electrode lines and acommon electrode potential (common electrode signal) is applied fromboth ends of each common electrode line. With the above, each pixelcircuit is formed so that a ratio L/W between a channel length L and achannel width W in the field-effect transistor increases with anincreasing electrical distance from inputting ends of each commonelectrode line. This reduces nonuniformity of the level shift in thepixel potential caused by difference in the amount of charge which movesto the voltage holding electrode due to parasitic capacities between thescanning signal line and the common electrode line as well as the signalpropagation delay characteristic in the common electrode lines, andtherefore uniformalizes the level shift distribution.

According to the sixteenth aspect of the present invention, one of theelectrostatic capacities which are formed between the drain electrode ofthe field-effect transistor or the voltage holding electrode and otherelectrodes other than an electrostatic capacity Cgd between the gateelectrode and the drain electrode of the field-effect transistordecreases with an increasing electrical distance from an inputting endof the common electrode line. This reduces nonuniformity of the levelshift in the pixel potential caused by difference in the amount ofcharge which moves to the voltage holding electrode due to parasiticcapacities between the scanning signal line and the common electrodeline as well as the signal propagation delay characteristic in thecommon electrode lines, and therefore uniformalizes the level shiftdistribution.

The seventeenth aspect of the present invention covers a case in whichan active matrix substrate is formed with common electrode lines and acommon electrode potential (common electrode signal) is applied fromboth ends of each common electrode line. With the above, one of theelectrostatic capacities which are formed between the drain electrode ofthe field-effect transistor or the voltage holding electrode and otherelectrodes other than an electrostatic capacity Cgd between the gateelectrode and the drain electrode of the field-effect transistordecreases with an increasing electrical distance from inputting ends ofthe common electrode line (where common electrode potential is applied).This reduces nonuniformity of the level shift in the pixel potentialcaused by difference in the amount of charge which moves to the voltageholding electrode due to parasitic capacities between the scanningsignal line and the common electrode line as well as the signalpropagation delay characteristic in the common electrode lines, andtherefore uniformalizes the level shift distribution.

According to the eighteenth or the nineteenth aspect of the presentinvention, level shift non-uniformity of the pixel potential is reducedas in the first through the seventeenth aspects of the presentinvention. In addition, by controlling a speed of potential change in atransition from a scanning signal ON voltage to an OFF voltage outputtedfrom the scanning signal line drive circuit, it becomes possible tosubstantially uniformalize the speed of potential change in eachlocation on the scanning signal line. This eliminates or reducesnonuniformity of the level shift in the pixel potential caused by thesignal propagation delay characteristic in the scanning signal lines.

According to the twentieth aspect of the present invention, level shiftnon-uniformity of the pixel potential is reduced as in the first throughthe seventeenth aspects of the present invention, enabling to providehigh-quality images with reduced flickers, etc.

According to the twenty-first or the twenty-second aspect of the presentinvention, level shift non-uniformity of the pixel potential is reducedas in the eighteenth or the nineteenth aspects of the present invention,enabling to provide high-quality images with reduced flickers etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an overall configuration of a liquidcrystal display device which uses a TFT substrate as an active matrixsubstrate according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a pixel circuitaccording to the first embodiment.

FIG. 3 is a block diagram showing an example of scanning signal linedrive circuit according to the first embodiment.

FIG. 4 is a schematically simplified voltage waveform chart of a signaland a pixel potential for driving a conventional TFT substrate as wellas TFT substrates according to the first and the second embodiments.

FIG. 5 is an equivalent circuit diagram showing a signal propagationpath of the scanning signal in the active matrix substrate according tothe first embodiment, with attention focused on signal propagation delayin one scanning signal line.

FIG. 6 is a waveform chart showing voltage waveforms and currentwaveforms in primary portions according to the first embodiment.

FIG. 7-(A) shows a distribution of pixel potentials before applicationof the configuration according to the first embodiment. FIG. 7-(B) showsa distribution of capacities between the scanning signal line and thepixel electrode according to the first embodiment. FIG. 7-(C) shows adistribution of pixel potentials after application of the configurationaccording to the first embodiment.

FIG. 8 shows a result of simulation on pixel potential distributionaccording to the first embodiment.

FIG. 9 is a block diagram showing an overall configuration of a liquidcrystal display device which uses a TFT substrate as an active matrixsubstrate according to a second and a third embodiments of the presentinvention.

FIG. 10 is a circuit diagram showing a configuration of a pixel circuitin the TFT substrate as an active matrix substrate according to thesecond and the third embodiments.

FIG. 11 is an equivalent circuit diagram showing propagation paths ofscanning signals and a common electrode signal in the active matrixsubstrate according to the second and the third embodiments, withattention focused on signal propagation delay in one scanning signalline and one common electrode line.

FIG. 12 is a waveform chart showing voltage waveforms and currentwaveforms in primary portions according to the second embodiment.

FIG. 13-(A) shows a distribution of pixel potentials before applicationof the configuration according to the second embodiment. FIG. 13-(B)shows a distribution of capacities between the scanning signal line andthe pixel electrode according to the second embodiment. FIG. 13-(C)shows a distribution of pixel potentials after application of theconfiguration according to the second embodiment.

FIG. 14 is a block diagram showing a configuration of a scanning signalline drive circuit in a liquid crystal display device which uses anactive matrix substrate according to the third embodiment of the presentinvention.

FIG. 15 is a schematically simplified voltage waveform chart of a signaland a pixel potential for driving a TFT substrate according to the thirdembodiment.

FIG. 16 is a waveform chart showing voltage waveforms and currentwaveforms in primary portions according to the third embodiment.

FIG. 17-(A) shows a distribution of pixel potentials before applicationof the configuration according to the third embodiment. FIG. 17-(B)shows a distribution of capacities between the scanning signal line andthe pixel electrode according to the third embodiment. FIG. 17-(C) showsa distribution of pixel potentials after application of theconfiguration according to the third embodiment.

FIG. 18 is an explanatory diagram illustrating that a thin-filmtransistor has a Vgs-Vds-Id characteristic (gate-to-sourcevoltage/drain-to-source voltage/drain current characteristic).

FIG. 19 is a circuit diagram showing a configuration of a pixel circuitin a TFT substrate as a conventional active matrix substrate.

FIG. 20 is a circuit diagram for describing an application of thepresent invention to an organic EL display device.

FIG. 21 is a circuit diagram for describing an application of thepresent invention to an organic EL display device.

LEGEND

-   -   100 TFT substrate (active matrix substrate)    -   101 Opposed substrate    -   102 Thin-film transistor (TFT)    -   103 Pixel electrode (voltage holding electrode)    -   200 Data signal line drive circuit    -   300 Scanning signal line drive circuit    -   CS Common electrode line drive circuit    -   Ec Opposed electrode    -   P(i, j) Pixel circuit (i=1 to N, j=1 to M)    -   CS(j) Common electrode line (j=1 to M)    -   G(j) Scanning signal line (j=1 to M)    -   S(i) Data signal line (i=1 to N)    -   VD1 Input terminal    -   VD2 Input terminal    -   3 a Shift register section    -   3 b Selector switch (switch section)    -   GCK Clock signal    -   GSP Data signal (start pulse signal)    -   SC Slew-rate control circuit (gradient control section)    -   Cgd Capacity between scanning signal line and pixel electrode    -   Ccs Capacity between common electrode and pixel electrode        (supplemental capacity)    -   Clc Capacity between opposed electrode and pixel electrode        (liquid-crystal capacity)    -   Cpix Pixel capacity (voltage holding capacity)    -   Id(i, j) Waveform of current passing through TFT (i=1 to N, j=1        to M)    -   Vcs Common electrode potential    -   Vcs(i, j) Waveform of common electrode potential (i=1 to N, j=1        to M)    -   Vcom Opposed-electrode potential    -   Vd (i, j) Pixel potential (potential of the pixel electrode)        (i=1 to N, j=1 to M)    -   Vdp Pixel potential (when positive voltage is applied)    -   Vdn Pixel potential (when negative voltage is applied)    -   Vg(j) Scanning signal (j=1 to M)    -   Vg(i, j) Voltage waveform of the scanning signal (i=1 to N, j=1        to M)    -   Vgl Gate-off voltage    -   Vgh Gate-on voltage    -   Vth TFT threshold voltage    -   Vs(i) Data signal (i=1 to N)    -   Vsp Data signal voltage (when positive voltage is applied)    -   Vsn Data signal voltage (when negative voltage is applied)    -   ΔVd(i, j) Pixel potential level shift (i=1 to N, j=1 to M)    -   ΔVdp(i, j) Pixel potential level shift (when positive voltage is        applied) (i=1 to N, j=1 to M)    -   ΔVdn (i, j) Pixel potential level shift (when negative voltage        is applied) (i=1 to N, j=1 to M)    -   ΔQd (i, j) Charge amount transferred from start to end of        scanning signal trailing end 0=1 to N, j=1 to M)

BEST MODE FOR CARRYING OUT THE INVENTION 0. Basic Study

<0.1 Pixel Circuit and Signal Propagation Path>

Before describing the embodiments of the present invention, descriptionwill cover a basic study conducted by the inventor of the presentinvention in order to accomplish the objects of the present invention.The basic study takes a case of a TFT substrate 100 which is an activematrix substrate configured as shown in FIG. 9. The TFT substrate 100includes a transparent insulating substrate such as glass, on which aplurality (M) of scanning signal lines G(1) through G(M) and a plurality(N) of data signal lines S(1) through S(N) are formed to cross with eachother. Further, correspondingly to each of the intersections, a pixelcircuit P(i, j) (i=1 to N, j=1 to M) is formed in a matrix pattern as aplurality (M×N) of pixel formation regions. Also, a plurality of commonelectrode lines CS(1) through CS(M) are formed, each in parallel to acorresponding one of the scanning signal lines G(1) through G(M).

As shown in FIG. 10, each pixel circuit P(i, j) includes: a TFT 102,i.e. a field-effect transistor serving as a switching element, which hasits source electrode connected with the data signal line S(i) thatpasses the corresponding intersection, and its gate electrode connectedwith the scanning signal line G(j) that passes the correspondingintersection; and a pixel electrode 103 connected with the drainelectrode of the TFT 102. The pixel electrode 103 and the opposedelectrode Ec, which is formed on the entire surface of the opposedsubstrate, form a liquid-crystal capacity Clc. The pixel electrode 103and common electrode line CS(j) form a common-electrode capacity Ccs,and the pixel electrode 103 and the scanning signal line G(j) form aparasitic capacity Cgd.

In general, signal lines formed on a transparent insulating substratesuch as glass, e.g. the scanning signal lines G(1) through G(M) and thecommon electrode line CS(1) through CS(M) formed on the TFT substrate100, are difficult to form as idealized signal lines which are free fromsignal propagation delay. So, the lines have a certain degree of signalpropagation delay characteristic. FIG. 11 is an equivalent circuitdiagram showing propagation paths of a scanning signal and a commonelectrode signal, with attention focused on signal propagation delay inone scanning signal line G(j) and one common electrode line CS(j). InFIG. 11, resistors rg1, rg2, . . . , rgi, . . . , rgN each represent aresistance component of the scanning signal line G(j) for a pixelcircuit, and its resistance value is determined primarily by thematerial of the wiring, the width of the wiring and the length of thewiring in the scanning signal line G(j). Resistors rc1, rc2, . . . ,rci, . . . , rcN each represent a resistance component of the commonelectrode line CS(j) for a pixel circuit, and its resistance value isdetermined primarily by the material of the wiring, the width of thewiring and the length of the wiring in the common electrode line CS(j).

The inventor of the present invention discovered, through simulations,etc., that in the study of the above-described signal propagation, caremust also be paid to influences from the parasitic capacity which ispresent between the scanning signal line and the common electrode lineas shown in FIG. 11. In FIG. 11, capacities cgc1, cgc2, . . . , cgci, .. . , cgcN each represent a parasitic capacity per a pixel circuitresulting from a single-tier or multi-tier capacity-couplingrelationship between the scanning signal line(s) and the commonelectrode line. For example, such a capacity results from a seriescapacity coupling of a capacity Cgd which is a capacity between thescanning signal line and the pixel electrode with a capacity Ccs whichis a capacity between the pixel electrode and the common electrode line.Likewise, capacities cg1, cg2, . . . , cgi, . . . , cgN representcapacities obtained by subtracting the parasitic capacities cgc1 throughcgcN, from various parasitic capacities formed by the scanning signalline and other electrodes/signal-lines which are in thecapacity-coupling relationship with the scanning signal lines, and isconstituted by e.g. a cross capacity resulting from crossing of ascanning signal line with a data signal line. Thus, the scanning signallines and the common electrode lines are distributed-constant, delayed,signal-propagation paths.

<0.2 Voltage Waveform at Various Points>

As shown in FIG. 9, a liquid crystal display device which uses theabove-described active matrix substrate provided by the TFT substrate100 includes: a data signal line drive circuit 200 which applies datasignals Vs(1) through Vs(N) to the data signal lines S(1) through S(N)respectively; a scanning signal line drive circuit 300 which appliesscanning signals Vg(1) through Vg(M) to the scanning signal lines G(1)through G(M) respectively; two common electrode line drive circuits CSwhich give a common electrode potential Vcs to the common electrodelines CS(1) through CS(M), respectively from one end and the other endof the lines; and an opposed electrode drive circuit COM which gives anopposed-electrode potential Vcom to the opposed electrodes Ec.

FIG. 4-(A) shows a voltage waveform of a scanning signal Vg(j) appliedfrom the scanning signal line drive circuit 300 to the scanning signalline G(j). FIG. 4-(B) shows a voltage waveform of a data signal Vs(i)applied from the data signal line drive circuit 200 to the data signalline S(i). FIG. 4-(C) shows a voltage waveform of a common electrodepotential Vcs and an opposed-electrode potential Vcom given from thecommon electrode line drive circuit CS and the opposed electrode drivecircuit COM to the common electrode line CS(j) and the opposed electrodeEc respectively. FIG. 4-(D) shows a voltage waveform of a pixelpotential Vd (i, j) of the pixel circuit P(i, j) in the TFT substrate100 shown in FIG. 9.

FIG. 12-(A) shows how the scanning signal Vg(j), which is applied fromthe scanning signal line drive circuit 300 to the scanning signal lineG(j), is deformed in the panel (in the TFT substrate 100) due to thesignal propagation delay characteristic (FIG. 11) of the scanning signalline G(j). FIG. 12-(B) shows how the potential Vcs(j) of the commonelectrode line CS(j) is deformed in the panel (in the TFT substrate 100)due to an influence from the parasitic capacity which is present betweenthe scanning signal line G(j) and the common electrode line CS(j). Itshould be appreciated that, in the FIGS. 12-(A) and 12-(B), a symbol“Vg(i, j)” indicates a voltage waveform of the scanning signal Vg(j) inthe pixel circuit P(i, j), whereas a symbol “Vcs(i, j)” indicates avoltage waveform of the common electrode potential Vcs in the pixelcircuit P(i, j).

As shown in FIG. 12-(A), a voltage waveform Vg(1, j) of the scanningsignal which is the voltage waveform of the potential at the gateelectrode g(1, j) of the TFT in the pixel circuit P(1, j) (See FIG. 11)right after the output from the scanning signal line drive circuit 300shows very little deformation. On the contrary, due to theabove-described signal propagation delay characteristic, a voltagewaveform Vg(n, j) of the scanning signal in an intermediate portion ofthe scanning signal line G(j) (at a center region of the TFT substrate100) shows a deformation to a certain extent, whereas a voltage waveformVg(N, j) near the terminating end of the scanning signal line G(j) showsa deformation to a grater extend.

As shown in FIG. 12-(B), there is no major waveform change in voltagewaveforms Vcs(1, j) and Vcs(N, j) of the common electrode potential Vcsright after the output from the two common electrode line drive circuitsCS. On the contrary, there is a big change in a voltage waveform Vcs(n,j) of the common electrode potential Vcs in a center portion of thecommon electrode line CS(j) (at a center region of the TFT substrate100), due to an influence from the parasitic capacity between thescanning signal line G(j) and the common electrode line CS(j) as well asan influence from the signal propagation delay characteristic. Theinventor of the present invention discovered this from a study based onthe equivalent circuit in FIG. 11, computer simulation and so on.

<0.3 Pixel Potential Level Shift>

The TFT 102 in each pixel circuit P(i, j) of the TFT substrate 100 as anactive matrix substrate shown in Fit. 9 is not a perfect ON/OFF switch,but has a gate-to-source voltage/drain-to-source voltage/drain currentcharacteristic (hereinafter called Vgs-Vds-Id characteristic) as shownin FIGS. 18-(A) and 18-(B). In FIG. 18-(A), the horizontal axisrepresents a voltage Vgs applied between the gate and the source of theTFT, whereas the vertical axis represents a drain current Id. In FIG.18-(B), the horizontal axis represents a voltage Vds applied between thedrain and the source of the TFT, whereas the vertical axis represents adrain current Id. The inventor of the present invention discovered thatdue to the Vgs-Vds-Id characteristic described above, variation arepresent in the level shift ΔVd of the pixel potential Vd; specifically,that the level shift ΔVd varies depending on the location of pixelcircuit P(i, j) (hereinafter this will be referred to as “nonuniformityof the level shift ΔVd”. Hereinafter, description will cover thenonuniformity of the level shift ΔVd.

Normally, a scanning pulse which serves as the scanning signal Vg(j) isa pulse whose potential changes between a voltage which is sufficient toturn ON the TFT (hereinafter called “gate-on voltage”) Vgh and a voltagewhich is sufficient to turn OFF the TFT (hereinafter called “gate-offvoltage”) Vgl. As shown in FIG. 18-(A), from the time when the scanningsignal given to the gate electrode of the TFT starts to fall from thegate-on voltage Vgh toward the gate-off voltage Vgl to the time when thesignal comes completely down to the gate-off voltage level Vgl, there isa region from the gate-on voltage Vgh to near the TFT threshold voltageVth which works as an intermediate ON region.

As shown in FIG. 12-(A), in the pixel circuit P(1, j) which is locatedright after the output point from the scanning signal line drive circuit300, or more specifically, in the pixel circuit P(1, j) which is locatednear an inputting end of the scanning signal Vg(j) to the scanningsignal line G(j) (Hereinafter, simply called “near an inputting end”),the scanning signal Vg(j) falls instantaneously from the gate-on voltageVgh to the gate-off voltage level Vgl, so the characteristic of theintermediate ON region of the TFT has little effect. Generally, theamount of pixel potential change ΔVd1 due to capacity coupling can beexpressed as follows, with the capacity between the scanning signal lineG(j) and the pixel electrode represented by Cgd, and the pixel capacityrepresented by Cpix, and Vgpp=Vgl−Vgh:ΔVd1=Vgpp·Cgd/Cpix  (1)From the equation (1), the level shift ΔVd(1, j) which occurs in thepixel potential Vd(1, j) near the inputting end can be approximated as:ΔVd(1,j)=Vgpp·Cgd/Cpix

Likewise, a level shift ΔVd(n, j) occurs in the pixel potential Vd(n, j)of the pixel circuit P(n, j) which is located away from the scanningsignal line drive circuit 300 and near the center of the scanning signalline G(j) (hereinafter, simply called “near the center”), and also, alevel shift ΔVd(N, j) occurs in the pixel potential Vd(N, j) of thepixel circuit P(N, j) which is located near the terminating end of thescanning signal line G(j) (hereinafter, simply called “near theterminating end”). However, the scanning signal voltage waveform Vg(n,j) near the center and the scanning signal voltage waveform Vg(N, j)near the terminating end are deformed in their falling edge, andtherefore, affected by the above-described characteristic of theintermediate ON region of the TFT, resulting in reduced level shift inthe pixel potential Vd (The absolute value becomes smaller). Therefore,the level shift ΔVd(n, j) near the center and the level shift ΔVd(N, j)near the terminating end are expressed as follows:|ΔVd(n,j)|<|Vgpp·Cgd/Cpix|,|ΔVd(N,j)|<|Vgpp·Cgd/Cpix|This means that there are level shift differences between the regionnear the inputting end (right after the output from the scanning signalline drive circuit 300) and the other regions: namely,|ΔVd(n,j)|<|ΔVd(1,j)|,|ΔVd(N,j)|<|ΔVd(1,j)|In this way, there exists a nonuniformity of the level shift ΔVd, whichwill be detailed hereinafter using the equations and the drawings.

Attention will be focused to the scanning signal which is given to thegate electrode of each TFT (hereinafter called “gate signal”), and atime period between the time point to when the signal starts to fall anda later time point t: For the voltage of the gate signal (hereinaftercalled “gate voltage”), the amount of shift will be represented byΔVg(t). The amount of shift in the common electrode potential Vcs willbe represented by ΔVcs(t). The amount of shift in the opposed-electrodepotential Vcom will be represented by ΔVcom(t). The capacity between thegate and the drain will be represented by Cgd, the capacity between thepixel electrode and the common electrode line will be represented byCcs, the capacity between the pixel electrode and the opposed electrode(liquid-crystal capacity) will be represented by Clc, the pixel capacitywill be represented by Cpix(=Clc+Ccs+Cgd), the current which passes fromthe data signal line to the pixel electrode through the TFT will berepresented by Id(t), and the amount of charge given by the currentId(t) to the pixel electrode will be represented by ΔQd(t). With theabove, the level shift ΔVd(t) which occurs in the pixel potential Vd atthe time point t can be expressed in the following equation (2):

$\begin{matrix}{{\Delta\;{{Vd}(t)}} = {{\Delta\;{{{Vg}(t)} \cdot {Cgd}}\text{/}{Cpix}} + {\Delta\;{{{Vcs}(t)} \cdot {Ccs}}\text{/}{Cpix}} + {\Delta\;{{{Vcom}(t)} \cdot {Clc}}\text{/}{Cpix}} + {\Delta\;{{Qd}(t)}\text{/}{Cpix}}}} & (2)\end{matrix}$Also, the current Id(t) which flows at the time point t in theabove-described intermediate ON region is determined by the gate-sourcevoltage Vgs(t) and the drain-source voltage Vds(t), and by theVgs-Vds-Id characteristic shown in FIG. 18-(A) and FIG. 18-(B). Thecharge amount ΔQd(t) which is given to the pixel electrode by thecurrent flowing through the TFT from the time point ta when the gatesignal starts to fall to the time point t is obtained as an integrationvalue of the current Id(t) which flows through the TFT from the time tato the time t.

In the above, the gate-source voltage Vgs(t) and the drain-sourcevoltage Vds(t) of the TFT at the time point t satisfy the followingrelationship, where Vg(t) represents the gate voltage, and Vs(t)represents the source voltage (the voltage of the data signal) at thetime point t:Vgs(t)=Vg(t)−Vs  (3)Vds(t)=ΔVd(t)  (4)

In this way, the charge amount ΔQd(t) is determined uniquely by theVgs-Vds-Id characteristic expressed in the equations (2) through (4) aswell as FIG. 18-(A) and FIG. 18-(B). Specifically, the charge amount ΔQdwhich is given to the pixel electrode by the current flowing through aTFT from the time when the gate voltage of the TFT starts to fall to thetime when the voltage has fallen down completely is determined uniquely.

Now, assume that the gate voltage in each TFT has fallen downcompletely, and then sufficient time has elapsed to come to a time pointtb. At this particular time point tb,ΔVg(t)=Vgpp=Vgl−Vgh,ΔVcs(t)=0,ΔVcom=0Therefore, the level shift ΔVd can be expressed as:ΔVd=Vgpp·Cgd/Cpix+ΔQd/Cpix  (5)

Due to the signal propagation delay characteristic of the scanningsignal line G(j), the Vg(t) takes different values in different pixelcircuit P(i, j): In a pixel circuit P(i, j) which is away from thescanning signal line drive circuit 300, the gate-source voltage Vgs(t)has a long period in which the voltage value is not smaller than the TFTthreshold voltage Vth, and a large charge amount ΔQd transfers to thepixel electrode via the TFT (Note: Vgpp<0, ΔQd>0 and ΔVd<0). For thisreason, the level shift ΔVd of the pixel potential Vd is reduced (theabsolute value |ΔVd| is decreased). Likewise, due to the signalpropagation delay characteristic of the common electrode line CS(j), theamount of shift ΔVcs(t) of the common electrode potential Vcs takesdifferent values in different pixel circuit P(i, j): in a pixel circuitP(i, j) which is away from the common electrode line drive circuit CS,the ΔVcs(t) is large and the transferred charge amount ΔQd is large.This, too, reduces the level shift ΔVd of the pixel potential Vd (theabsolute value |ΔVd| is decreased).

In this way, the level shift ΔVd in the pixel potential Vd is notuniform within the TFT substrate 100, due to the signal propagationdelay characteristics of the scanning signal line and the commonelectrode line and the TFT characteristic (FIG. 11, FIGS. 18-(A) and18-(B)) in the TFT substrate 100 which serves as the active matrixsubstrate. Then, with increasing size of the screen and increasing levelof fineness of the display device which uses this TFT substrate 100, thenonuniformity issue eventually comes to a point where it is no longernegligible.

The present invention was made on the basis of the above-described studyand findings (discoveries) by the inventor of the present invention,with an object of eliminating or reducing the nonuniformity of the levelshift ΔVd. Specifically, based on the equation (5), according to thepresent invention, formation of pixel circuits in the TFT substrate 100is performed in such a way that each pixel circuit P(i, j) will havesubstantially equal (Vgpp·Cgd+ΔQd)/Cpix. In more specific words, asdescribed in the following embodiments, etc., various electrostaticcapacities (such as the capacity Cgd between the scanning signal lineG(j) and the pixel electrode) and TFT characteristic in each pixelcircuit P(i, j) of the TFT substrate 100 are varied in accordance withthe location of the pixel circuit P(i, j), so that all the pixelcircuits in the TFT substrate 100 will have substantially the same valuefor (Vgpp·Cgd+ΔQd)/Cpix. Hereinafter, embodiments of the presentinvention incorporating such an arrangement as the above will bedescribed with reference to the attached drawings. It should beappreciated that the charge amount ΔQd in the equation (5) is determinedby the equations (2) through (4) and the Vgs-Vds-Id characteristic shownin FIGS. 18-(A) and 18-(B) as described earlier: Therefore, if the TFTsubstrate 100 is formed with common electrode lines, consideration mustalso be made, in addition to the parasitic capacity between the scanningsignal line and the pixel electrode as well as the signal propagationdelay characteristic of the scanning signal line, to the parasiticcapacity between the scanning signal line and the common electrode lineas well as the signal propagation delay characteristic of the commonelectrode line (See FIG. 11).

1. First Embodiment

FIG. 1 is a block diagram showing an overall configuration of a liquidcrystal display device which uses a TFT substrate as an active matrixsubstrate according to a first embodiment of the present invention. Theliquid crystal display device include: a liquid crystal display panel 1;a drive circuit including a data signal line drive circuit 200, ascanning signal line drive circuit 300 and an opposed electrode drivecircuit COM; and a control circuit 600.

The liquid crystal display panel 1 is made of a pair of electrodesubstrate sandwiching a liquid crystal layer. Each electrode substratehas its outer surface covered by a polarizer. Of the pair of electrodesubstrates, one is an active matrix substrate called TFT substrate. TheTFT substrate 100 includes an insulating substrate made of glass forexample, formed with data signal lines S(1)-S(N) and scanning signallines G(1)-G(M) crossing with each other in a grid pattern. Further,correspondingly to each of the intersections made by the data signallines S(1)-S(N) and the scanning signal lines G(1)-G(M), a plurality(N×M) of pixel circuits P(i, j) are formed in a matrix pattern. Each ofthe pixel circuits P(i, j) corresponds to one of pixels which constituteimages to be displayed. These signal lines S(1)-S(N), G(1)-G(M) andpixel circuits P(i, j) are covered virtually entirely by an alignmentfilm. On the other hand, the other of the pair of electrode substratesis called opposed substrate, provided by a transparent insulatingsubstrate such as glass, and has its entire surface laminated with anopposed electrode and then with an alignment film. Note that the presentembodiment has a different configuration from the one in FIG. 9 whichwas described in the basic study, in that the TFT substrate 100 is notformed with common electrode lines.

Each pixel circuit P(i, j) includes a switching element provided by afield-effect transistor TFT 102, and a pixel electrode 103 connectedwith the data signal line S(i) via the TFT 102, having a circuitconfiguration as shown in FIG. 2. Specifically, each pixel circuit P(i,j) includes: a TFT 102 serving as a switching element, having its sourceelectrode connected with the data signal line S(i) which passes thecorresponding intersection, and its gate electrode connected with thescanning signal line G(j) which passes the corresponding intersection;and a pixel electrode 103 connected with the drain electrode of the TFT102. The pixel electrodes 103 and the opposed electrode Ec which isformed on the entire surface of the opposed substrate 101 form aliquid-crystal capacity Clc. The pixel electrode 103 and the scanningsignal line G(j) form a parasitic capacity Cgd. It should be appreciatedthat in the present embodiment, the pixel capacity Cpix, which is acapacity of capacitors formed by the pixel electrode 103 and otherelectrodes, and is a capacity for holding a voltage that represents thepixel value, includes the liquid-crystal capacity Clc and the parasiticcapacity Cgd.

The control circuit 600 generates control signals for controlling thedata signal line drive circuit 200, the scanning signal line drivecircuit 300, etc. The data signal line drive circuit 200 receivescontrol signals generated by the control circuit 600 and image signalsfrom an outside source, and based on these, generates data signalsVs(1)-Vs(N) as analog voltages, and then applies these data signalsVs(1)-Vs(N) respectively to data signal lines S(1)-S(N) which are formedin the TFT substrate 100 of the liquid crystal display panel 1. Thescanning signal line drive circuit 300 selects the scanning signal linesG(1)-G(M) in the liquid crystal display panel sequentially for eachhorizontal scanning period, and applies an active scanning signal (avoltage for turning ON the TFT in the pixel circuit) to the selectedscanning signal line, in each frame period (each vertical scanningperiod) for displaying an image on the liquid crystal display panel. Theopposed electrode drive circuit COM applies a signal which gives anelectric potential that serves as a baseline for the voltage to beapplied to the liquid crystal layer of the liquid crystal display panel1 to the opposed electrode Ec which is formed on the entire surface ofthe opposed substrate 101.

FIG. 3 is a block diagram which shows a configuration example of thescanning signal line drive circuit 300. In this example, the scanningsignal line drive circuit 300 includes: a shift register section 3 aprovided by a plurality (M) of cascade-connected flip-flops F(1), F(2) .. . F(j), . . . F(M); and selector switches 3 b each changing states inaccordance with an output from a corresponding flip-flop. Each selectorswitch 3 b has an input terminal VD1 supplied with a gate-on voltage Vghwhich is sufficient to turn ON the TFT 102 (See FIG. 1), and anotherinput terminal VD2 supplied with a gate-off voltage Vgl which issufficient to turn OFF the TFT 102. Therefore, in accordance with clocksignals GCK supplied to each of the flip-flops F(1)-F(M), a data signal(start pulse signal) GSP inputted to the first flip-flop F(1) istransferred sequentially over each of the flip-flops F(1)-F(M), andoutputted sequentially to each selector switch 3 b. In response to this,each selector switch 3 b outputs a gate-on voltage Vgh which turns ONthe TFT 102, for a period of one selected scanning period (TH) to thescanning signal line G(j), and thereafter, outputs a gate-off voltageVgl which turns OFF the TFT 102 to the scanning signal line G(j).Through this operation, the data signals Vs(1)-Vs(N) outputted from thedata signal line drive circuit 200 to the corresponding data signallines S(1)-S(N) (See FIG. 1) are respectively written to thecorresponding pixel circuits P(i, j) (i.e. to the pixel capacitythereof).

When the TFT substrate 100 is driven as described above, schematicallysimplified voltage waveforms of the scanning signal Vg(j), the datasignal Vs(i), the common electrode potential Vcs, the opposed-electrodepotential Vcom, and the pixel potential (the potential of the pixelelectrode) Vd (i, j) are as shown in FIGS. 4-(A) through 4-(D), i.e.generally the same as the waveforms in the convention described earlier,so no more description will be given here. However, these voltagewaveforms are different in details from the convention, which will bedescribed later.

Following the steps described above, a plurality of data signalsVs(1)-Vs(N) are applied to the data signal lines S(1)-S(N) respectively,and a plurality of scanning signals Vg(1)-Vg(M) are applied to thescanning signal lines G(1)-G(M) respectively, whereby the pixelelectrode 103 in each pixel circuit P(i, j) of the liquid crystaldisplay panel 1 is given, via the TFT 102, a voltage representing thevalue of the pixel for the image to be displayed, with a baselineprovided by the potential Vcom of the opposed electrode Ec, and thisvoltage is held in the pixel capacity in each pixel circuit P(i, j).Thus, a voltage which is equal to the potential difference between eachpixel electrode 103 and the opposed electrode Ec is applied to theliquid crystal layer. By controlling optical transmittance based on theapplied voltage, the liquid crystal display panel 1 displays an imagerepresented by the image signal received from e.g. an outsidesignal-source.

FIG. 5 is an equivalent circuit diagram showing a propagation path of ascanning signal, with attention focused on signal propagation delay in ascanning signal line G(j). The present embodiment (See FIG. 1) has aconfiguration which differs from the one shown in FIG. 9, i.e. there isno common electrode line. Therefore, the signal propagation delaycharacteristic in each scanning signal line G(j) can be evaluated on thebasis of the equivalent circuit shown in FIG. 5, and the findings fromthe basic study are applicable to the present embodiment, except for theinfluence from the parasitic capacity and potential changes related tothe common electrode lines. It should be appreciated that in FIG. 5,resistors rg1, rg2, . . . , rgi, . . . , rgN each represent a resistancecomponent of the scanning signal line G(j) for a pixel circuit, and itsresistance value is determined primarily by the material of the wiring,the width of the wiring and the length of the wiring in the scanningsignal line G(j). Capacities cg1, cg2, . . . , cgi, . . . , cgc eachrepresent various parasitic capacities for a pixel circuit, formedbetween the scanning signal line G(j) and other electrodes, signallines, etc. which have a capacity coupling relationship with thisscanning signal line. Hereinafter, description will cover details of thepresent embodiment which takes in account the signal propagation delaycharacteristic of the scanning signal line G(j) based on the equivalentcircuit shown in FIG. 5.

FIG. 6-(A) shows precise voltage waveforms (time course changes of avoltage) of a falling scanning signal at a gate electrode of the TFT 102in the pixel circuit P(i, j) of the TFT substrate 100 according to thepresent embodiment which is configured as described above. Vg(1, j),Vg(n, j), and Vg(N, j) represent waveforms of the scanning signal Vg(j)near the inputting end (right after an output from the scanning signalline drive circuit 300), near the center and near the terminating endrespectively, of the scanning signal line G(j). FIG. 6-(B) showselectric current waveforms (time course changes of the current) of thecurrent which passes through the TFT 102 of the pixel circuit P(i, j)when the scanning signal Vg(j) falls from the gate-on voltage Vgh to thegate-off voltage Vgl. Id(1, j), Id(n, j) and Id(N, j) representwaveforms of the current which passes through the TFT 102 near theinputting end, near the center and near the terminating endrespectively, of the scanning signal line G(j). FIG. 6-(C) representswaveforms of the potential of the pixel electrode 103 in the pixelcircuit P(i, j) at the time when the scanning signal Vg(j) falls fromthe gate-on voltage Vgh to the gate-off voltage Vgl. Vd (1, j), Vd (n,j) and Vd (N, j) represent waveforms of the potential near the inputtingend, near the center and near the terminating end respectively, of thescanning signal line G(j).

In the TFT substrate 100, the scanning signal Vg(j) is deformed in theTFT substrate 100 by the signal propagation delay characteristic of thescanning signal line G(j) in such a pattern as exemplified by the Vg(i,j) in FIG. 6-(A) (i=1, n, N).

Because of these influences from the Vg(i, j) and various TFTcharacteristics (FIGS. 18-(A) and 18-(B)), the waveform Id(i, j) of thecurrent which flows through the TFT 102 during the time when the gateelectrode voltage (gate voltage) in each TFT 102 is falling from thegate-on voltage Vgh to the gate-off voltage Vgl takes different forms asshown in FIG. 6-(B), depending on the location on the scanning signalline G(j) (More generally, depending on the location on the TFTsubstrate 100). Thus, the charge amount ΔQd(i, j) which transfers to thepixel electrode 103 via the TFT 102 during the time when the gatevoltage in each TFT 102 falls from the gate-on voltage Vgh to thegate-off voltage Vgl takes different values depending on the location onthe scanning signal line G(j). Therefore, if all pixel circuits P(i, j)have the same parasitic capacity Cgd between the scanning signal lineand the pixel electrode (between the gate electrode and the drainelectrode of the TFT 102) as in the conventional TFT substrate 100, thepotential waveform Vd (i, j) of the pixel electrode 103 changes as shownin FIG. 6-(C), depending on the location on the scanning signal lineG(j), due to the differences in charge transfer to the pixel electrodes103. As a result, even after a sufficient amount of time has passedsince the scanning signal Vg(j) fell down to the gate-off voltage Vgl,the level shift ΔVd(i, j) in the potential Vd(i, j) of each pixelelectrode takes different values depending on the location on thescanning signal line G(j), resulting in distribution nonuniformity ofthe level shift ΔVd, due to the differences of the charge amount ΔQd(i,j) based on the equation (5) discussed in the basic study. Specifically,the potential Vd(i, j) of the pixel electrode 103 changes as shown inFIG. 7-(A) depending on the location i on the scanning signal line G(j).In more specific words, the potential Vd(i, j) of the pixel electrode103 increases as the distance from the inputting end (scanning signalline drive circuit 300) increases, with the rate of increase decreasingas the distance from the inputting end increases. In response to this,the absolute value |ΔVd| of the level shift in the pixel potential Vddecreases as the distance from the inputting end increases, but the rateof decrease decreases as the distance from the inputting end increases.This is attributable to the fact that the propagation path for thescanning signal Vg(j) is a CR-distributed constant wiring in whichhigh-frequency components is lost increasingly as the distance from thescanning signal line drive circuit 300 increases. Computer simulationsgave a similar result as shown in FIG. 8.

The present embodiment takes care of the above-described nonuniformityof the pixel potential Vd(i, j) or the level shift ΔVd (FIG. 7-(A)), byforming each pixel circuit P(i, j) so that the parasitic capacity Cgdbetween the scanning signal line and the pixel electrode (between thegate electrode and the drain electrode of the TFT 102) in each pixelcircuit P(i, j) will be different in accordance with the location on thescanning signal line G(j) (more generally, the location in the TFTsubstrate 100) as shown in FIG. 7-(B). Specifically, each pixel circuitP(i, j) is formed in such a way that the parasitic capacity Cgd or thecorrection amount ΔCgd will be substantially equal to |ΔQd/Vgpp| (thecorrection amount ΔCgd refers to a capacity component of the parasiticcapacity Cgd to be varied in accordance with the location). Moreaccurately, values of the parasitic capacity Cgd are adjusted by meansof simulations, for example, so that (Vgpp·Cgd+ΔQd)/Cpix will beconstant. This means that each pixel circuit P(i, j) is formed in such away that the parasitic capacity Cgd will increase as the distance fromthe inputting end of the scanning signal line G(j) increases, but therate of the increase will become smaller as the distance from theinputting end increases. Thus, each pixel circuit P(i, j) is formed sothat its parasitic capacity Cgd will be larger as the circuit becomeselectrically farther away from the scanning signal line drive circuit300. As a result, as shown in FIG. 7-(C), it becomes possible that inall pixel circuits P(i, j), the potential Vd (i, j) of the pixelelectrode 103 and its level shift ΔVd have substantially the same valuesregardless of their location on the scanning signal line G(j) (thelocation in the TFT substrate 100), i.e. making possible to uniformalizethe distribution of level shift ΔVd. It should be appreciated that theparasitic capacity Cgd can be changed in accordance with its location onthe scanning signal line G(j) by changing the overlap area between thescanning signal line G(j) and the pixel electrode 103 and/or the overlaparea between the scanning signal line G(j) and the drain electrode ofthe TFT 102. In more specific words, the method disclosed in PatentDocument 4 (Japanese Patent Laid-Open Hei 11-84428 Gazette) can beutilized.

According to the present embodiment as described, nonuniformity of thelevel shift ΔVd is eliminated or reduced due to the arrangement that thepixel circuits P(i, j) are so formed that the parasitic capacity Cgdwill be different in accordance with the location on the scanning signalline G(j) correspondingly to the distribution of pixel potential Vd orof the level shift ΔVd. Thus, it is possible to provide high-qualityimages with reduced flickers etc. in a liquid crystal display devicewhich uses a TFT substrate according to the present embodiment.

It should be appreciated that Patent Document 4 (Japanese PatentLaid-Open Hei 11-84428 Gazette) discloses a technique which employs anarrangement that the capacity Cgd (Cgs) between the scanning signal lineand the pixel electrode is made smaller on the input side of thescanning signal line (gate signal line), and greater on the terminatingend side. This intends to uniformalize a level shift in the potentialcaused by capacity couplings due to nonuniform influences of the delayin scanning signal line to each pixel. However, no consideration is madeas to the level shift variation in each pixel potential in each TFTcaused by the change in charge amount which flows through the TFT fromthe time when the gate signal starts to fall to the time when the signalhas completely fallen down. Therefore, level shift non-uniformity of thepixel potential cannot be eliminated or reduced sufficiently byutilizing the technique disclosed in the Gazette alone.

2. The Second Embodiment

FIG. 9 is a block diagram showing an overall configuration of a liquidcrystal display device which uses a TFT substrate as an active matrixsubstrate according to a second embodiment of the present invention.This liquid crystal display device has the same configuration as theliquid crystal display device taken in the basic study which uses a TFTsubstrate as an active matrix substrate. Unlike the first embodiment,each of the scanning signal lines G(1)-G(M) formed on the TFT substrate100 is accompanied by a corresponding one of the common electrode linesCS(1)-CS(M) which lies in parallel thereto. With this arrangement, theembodiment includes two common electrode line drive circuits CS whichgive a common electrode potential Vcs to each of the common electrodelines CS(1)-CS(M), respectively from one and the other ends of thelines. Otherwise, the present embodiment is the same as the liquidcrystal display device (FIG. 1) which uses a TFT substrate 100 accordingto the first embodiment; therefore, identical elements will be indicatedwith the same reference symbols, and their details will not be repeated.

Each pixel circuit P(i, j) in the present embodiment includes afield-effect transistor, or a TFT 102 as a switching element, and apixel electrode 103 connected with the data signal line S(i) via the TFT102. The common electrode line CS(j) extends over the pixel electrode103 via an insulating layer, as illustrated in a circuit configurationin FIG. 10. Specifically, each pixel circuit P(i, j) includes: a TFT 102serving as a switching element, having its source electrode connectedwith the data signal line S(i) which passes the correspondingintersection, and its gate electrode connected with the scanning signalline G(j) which passes the corresponding intersection; and a pixelelectrode 103 connected with the drain electrode of the TFT 102. Thepixel electrodes 103 and the opposed electrode Ec formed on the entiresurface of the opposed substrate 101 form a liquid-crystal capacity Clc.The pixel electrodes 103 and the common electrode lines CS(j) form acommon electrode capacity Ccs. The pixel electrodes 103 and the scanningsignal lines G(j) form a parasitic capacity Cgd. It should beappreciated that in the present embodiment, the pixel capacity Cpix,which is a capacity of capacitors formed by the pixel electrode 103 andother electrodes, and is a capacity for holding a voltage thatrepresents the pixel value, includes the liquid-crystal capacity Clc,common-electrode capacity Ccs and the parasitic capacity Cgd.

An equivalent circuit of the signal propagation path of the scanningsignal and the common electrode signal according to the presentembodiment is as described in the basic study, i.e. a configurationshown in FIG. 11. FIG. 11 is an equivalent circuit diagram showingpropagation paths of the scanning signal and the common electrodesignal, with attention focused on signal propagation delay in onescanning signal line G(j) and one common electrode line CS(j).Hereinafter, description will cover details of the present embodimentwhich takes in account the signal propagation delay characteristic ofthe scanning signal G(j), with reference to the equivalent circuit shownin FIG. 11.

When the TFT substrate 100 having the configuration described above isdriven, schematically simplified voltage waveforms of the scanningsignal Vg(j), the data signal Vs(i), the common electrode potential Vcs,the opposed-electrode potential Vcom, and the pixel potential Vd (i, j)are as shown in FIGS. 4-(A) through 4-(D), i.e. generally the same asthe waveforms in the first embodiment and the convention describedearlier, so no more description will be given here. However, thesevoltage waveforms are different in details from the first embodiment,which will be described here below.

FIG. 12-(A) shows precise voltage waveforms of a falling scanning signalat a gate electrode of the TFT 102 in the pixel circuit P(i, j) of theTFT substrate 100 according to the present embodiment which isconfigured as described above. Vg(1, j), Vg(n, j), and Vg(N, j)represent voltage waveforms of the scanning signal Vg(j) near theinputting end, near the center and near the terminating end respectivelyof the scanning signal line G(j). FIG. 12-(B) shows potential waveformsVcs(i, j) in a part of common electrode line CS(j) which overlaps thepixel electrode 103 of the pixel circuit P(i, j), (more specifically,potential waveforms when the scanning signal Vg(j) falls from thegate-on voltage Vgh to the gate-off voltage Vgl). Vcs(1, j), Vcs(n, j)and Vcs(N, j) represent potential waveforms in the common electrode lineCS(j), near the inputting end, near the center and near the terminatingend respectively of the scanning signal line G(j). FIG. 12-(C)represents waveforms of the current Id(i, j) which flows through the TFT102 in the pixel circuit P(i, j) at the time when the scanning signalVg(j) falls from the gate-on voltage Vgh to the gate-off voltage Vgl.Id(1, j), Id(n, j) and Id(N, j) represent waveforms of the current nearthe inputting end, near the center and near the terminating endrespectively of the scanning signal line G(j). FIG. 12-(D) showspotential waveforms Vd(i, j) of the pixel electrode 103 in the pixelcircuit P(i, j) when the scanning signal Vg(j) falls from the gate-onvoltage Vgh to the gate-off voltage Vgl. Vd (1, j), Vd (n, j) and Vd (N,j) represent potential waveforms of the pixel electrode 103, near theinputting end, near the center and near the terminating end respectivelyof the scanning signal line G(j). It should be appreciated that in thefollowing embodiments, voltage waveforms, potential waveforms andcurrent waveforms on various locations on the scanning signal line G(j)will be indicated by using the same notation as has been used.

In the TFT substrate 100, the scanning signal Vg(j) is deformed in theTFT substrate 100 by the signal propagation delay characteristic of thescanning signal line G(j) in such a pattern as exemplified by Vg(i, j)in FIG. 12-(A) (i=1, n, N). As shown in FIG. 11, parasitic capacitiesare present between the common electrode line CS(j) and the scanningsignal line G(j), due to the intervention by the capacity Cgd formedbetween the scanning signal line and the pixel electrode and thecapacity Ccs formed between the pixel electrode and the common electrodeline. For this reason, the potential in the common electrode line CS(j)is affected by the voltage waveform Vg(i, j) of the scanning signal ineach pixel circuit P(i, j), and further, due to the signal propagationcharacteristic of the common electrode line CS(j), the potentialwaveform Vcs(i, j) in the common electrode line CS(j) changes as shownin FIG. 12-(B), depending on the location on the scanning signal lineG(j) (According to the present embodiment, this also is the location onthe common electrode line CS(j), and more generally is the location inthe TFT substrate 100).

Because of influences from the voltage waveform Vg(i, j) and thepotential waveform Vcs(i, j) as well as the TFT characteristicdifferences (FIGS. 18-(A) and 18-(B)), the waveform Id(i, j) of thecurrent which flows through the TFT 102 during the time when the gateelectrode voltage in each TFT 102 is falling from the gate-on voltageVgh to the gate-off voltage Vgl takes different forms as shown in FIG.12-(C), depending on its location on the scanning signal line G(j) or onthe common electrode line CS(j). Thus, the charge amount ΔQd(i, j) whichtransfers to the pixel electrode 103 via the TFT 102 during the timewhen the gate voltage in each TFT 102 falls from the gate-on voltage Vghto the gate-off voltage Vgl takes different values depending on thelocation on the scanning signal line G(j). Therefore, if all the pixelcircuits P(i, j) have the same parasitic capacity Cgd between thescanning signal line and the pixel electrode (between the gate electrodeand the drain electrode of the TFT 102) as in the conventional TFTsubstrate 100, the potential waveform Vd (i, j) of the pixel electrode103 changes as shown in FIG. 12-(D), depending on the location onscanning signal line G(j), due to the differences in charge transferamount to the pixel electrodes 103. As a result, even after a sufficientamount of time has passed since the scanning signal Vg(j) fell down tothe gate-off voltage Vgl, the level shift ΔVd (i, j) of the potential Vd(i, j) in each pixel electrode is different depending on the locationson the scanning signal line G(j), resulting in nonuniformity in thedistribution of the level shift tVd, due to the differences of thecharge amount ΔQd(i, j) based on the equation (5) which was discussed inthe basic study. Specifically, the potential Vd (i, j) of the pixelelectrode 103 changes as shown in FIG. 13-(A) depending on the locationon the scanning signal line G(j). In more specific words, the potentialVd (i, j) of the pixel electrode 103 increases as the distance from theinputting end (scanning signal line drive circuit 300) increases,attains a maximum value (peaks off) in a center portion, and thendecreases as the distance from the center portion increases toward theterminating end; however, the pixel potential Vd (N, j) near theterminating end does not drop as low as the pixel potential (1, j) nearthe inputting end. In response to these, the absolute value |ΔVd| of thelevel shift in the pixel potential Vd decreases as the distance from theinputting end increases, attains a minimum value near the center, andthen increases as the distance from the center portion increases towardthe terminating end. However, the absolute value |ΔVd(N, j)| of thelevel shift near the terminating end does not increase as to theabsolute value |ΔVd(1, j)| of the level shift near the inputting end.

A reason why the level shift absolute value |ΔVd| takes a minimum value(and the pixel potential Vd (i, j) takes a maximum value) in the centerportion of the scanning signal line G(j) is that the common electrodeline CS(j) is in parallel to the scanning signal line G(j), and thecommon electrode line CS(j) receives input from its two ends, i.e. thecommon electrode potential Vcs is applied by the common electrode linedrive circuits CS. Specifically, the potential Vcs(i, j) of the commonelectrode line CS(j) is influenced by the pulse fall of the scanningsignal Vg(j), and this influence becomes stronger as the electricaldistance from the two common electrode line drive circuits CS increases.Since two common electrode line drive circuits CS are connected with thetwo ends of common electrode line CS(j) respectively according to thepresent embodiment (FIG. 9), the influence is greater at locations whichare closer to the center of the scanning signal line G(j) (and thecenter also means the center in the common electrode line CS(j)).Therefore, the potential of the common electrode line CS(j) changes asshown in FIG. 12-(B) with the fall of the scanning signal Vg(j), makinga big transitional change near the center of the scanning signal lineG(j), and in response to this, drain-source voltage Vds of the TFTc 102in the pixel circuit P(n, j) near the center makes a large transitionalincrease. Therefore, not only due to the Vgs-Id characteristic of theTFT but also due to the Vds-Id characteristic of the TFT (See FIGS.18-(A) and 18-(B)), the drain current Id(n, j) of the TFT 102 in pixelcircuits P(n, j) near the center of scanning signal line G(j) increases,and the charge amount ΔQd which moves to the pixel electrode 103increases. As a result, the potential Vcs of the common electrode lineCS(j) exerts an influence to reduce the level shift absolute value |ΔVd|of the pixel potential Vd near the center. Thus, due to a combination ofthe influence from the voltage waveform Vg(i, j) in the scanning signalline G(j) and the influence from the potential waveform Vcs(i, j) in thecommon electrode line CS(j) as described, the potential Vd (i, j) of thepixel electrode 103 changes as shown in FIG. 13-(A), depending on thelocation on the scanning signal line G(j), causing a level ofnonuniformity in the level shift ΔVd, reflecting the change shown inFIG. 13-(A).

The present embodiment takes care of the above-described nonuniformityof the pixel potential Vd (i, j) or the level shift ΔVd, by forming eachpixel circuit P(i, j) so that the parasitic capacity Cgd between thescanning signal line and the pixel electrode (between the gate electrodeand the drain electrode of the TFT 102) in each pixel circuit P(i, j)will be different in accordance with the location on the scanning signalline G(j). Specifically, each pixel circuit P(i, j) is formed in such away that the parasitic capacity Cgd or the correction amount ACgd willbe substantially equal to |ΔQd/Vgpp|. More accurately, values of theparasitic capacity Cgd are adjusted by means of simulations, forexample, so that (Vgpp·Cgd+ΔQd)/Cpix will be constant. This means thatthe parasitic capacity Cgd will increase as the distance from theinputting end of the scanning signal line G(j) increases, attains amaximum value (peaks off) in the center portion, and then decreases asthe distance from the center portion increases toward the terminatingend; however, the parasitic capacity Cgd (N, j) near the terminating enddoes not decrease as low as the parasitic capacity (1, j) near theinputting end. Thus, each pixel circuit P(i, j) is formed so that itsparasitic capacity Cgd will be larger as the circuit becomeselectrically farther away from the scanning signal line drive circuit300 and as the circuit becomes electrically farther away from the commonelectrode line drive circuits CS. As a result, as shown in FIG. 13-(C),it becomes possible that in all pixel circuits P(i, j), the potential Vd(i, j) of the pixel electrode 103 and its level shift ΔVd havesubstantially the same values regardless of the location on the scanningsignal line G(j) (the location in the TFT substrate 100), i.e. makingpossible to uniformalize the distribution of level shift ΔVd. It shouldbe appreciated that the parasitic capacity Cgd can be changed inaccordance with its location on the scanning signal line G(j) bychanging the overlap area between the scanning signal line G(j) and thepixel electrode 103 and/or the overlap area between the scanning signalline G(j) and the drain electrode of the TFT 102. In more specificwords, the method disclosed in Patent Document 4 (Japanese PatentLaid-Open Hei 11-84428 Gazette) can be utilized.

According to the present embodiment as described, nonuniformity of thelevel shift ΔVd is eliminated or reduced in a TFT substrate 100, whichserves as an active matrix substrate, where common electrode lines CS(j)are formed in parallel to scanning signal lines G(j) and a commonelectrode potential Vcs is applied from two ends of each commonelectrode line CS(j), due to the arrangement that the pixel circuitsP(i, j) are so formed that the parasitic capacity Cgd will be differentin accordance with the location on the scanning signal line G(j)correspondingly to the distribution of pixel potential Vd or of thelevel shift ΔVd. Thus, it is possible to provide high-quality imageswith reduced flickers etc. in a liquid crystal display device which usesa TFT substrate according to the present embodiment.

3. Third Embodiment

Next, description will be made for a liquid crystal display device whichmakes use of a TFT substrate as an active matrix substrate according toa third embodiment of the present invention. The liquid crystal displaydevice includes a TFT substrate 100 which has generally the sameconfiguration as the one used in the second embodiment, or specificallythe TFT substrate 100 shown in FIG. 9, differing only in some details(such as the value of parasitic capacity Cgd). Further, this liquidcrystal display device has basically the same overall configuration asshown also in FIG. 9, so the same or corresponding elements will beindicated by the same reference symbols, and no details will be givenhereinafter. However, the present liquid crystal display device uses ascanning signal line drive circuit 300 which has a differentconfiguration (details to be described later) from that used in theliquid crystal display device which includes a TFT substrate 100according to the second embodiment.

Again, in the present embodiment, each pixel circuit P(i, j) is the sameas the pixel circuit P(i, j) in the second embodiment, and has thecircuit configuration as shown in FIG. 10. So, the same elements will beindicated by the same reference symbols, and no details will be givenhereinafter. Further, an equivalent circuit diagram of signalpropagation paths for the scanning signal and the common electrodesignal in the present embodiment is also the same as in the secondembodiment, and as shown in FIG. 11.

If the TFT substrate 100 which has the configuration described earlieris driven by the conventional scanning signal line drive circuit,voltage waveforms of the scanning signal Vg(j) will be as shown in FIG.12-(A), due to the signal propagation delay characteristic of thescanning signal line. On the contrary, according to the present liquidcrystal display device, the scanning signal line drive circuit 300 hasthe configuration disclosed in Patent Document 3 (Japanese PatentLaid-Open Hei 11-281957 Gazette), whereby the scanning signal line drivecircuit 300 outputs a scanning signal Vg(j) which has a controlledtrailing edge so that the trailing edge of the scanning signal voltagewaveform Vg(i, j) in all pixel circuits P(i, j) will have substantiallythe same gradient.

FIG. 14 is a block diagram showing the configuration of the scanningsignal line drive circuit 300 described above. The scanning signal linedrive circuit 300 has a similar configuration to the one in FIG. 3, andoperates basically in the same way, including: a shift register section3 a provided by a plurality (M) of cascade-connected flip-flops F(1),F(2) . . . F(j), . . . F(M); and selector switches 3 b each changingstates in accordance with an output from a corresponding flip-flop.However, as shown in FIG. 14, the scanning signal line drive circuit 300further includes, in the outputting stage, slew-rate control circuits(gradient control section) SC which are capable of controlling thefalling edge gradient of each output of the scanning signal Vg(j). Theslew-rate control circuit SC works as an equivalent to an outputcontrolling impedance device which controls the impedance of each outputfrom the scanning signal line drive circuit 300. The output impedance isincreased only when each scanning signal Vg(j) falls from the gate-onvoltage Vgh to the gate-off voltage Vgl. By proactively deforming thewaveform as outputted from the scanning signal line drive circuit 300,differences in the voltage falling speed in the TFT substrate 100 causedby waveform deformation due to the signal propagation delaycharacteristic of the scanning signal line G(j) (voltage falling speeddifferences at different locations on the scanning signal line G(j)) areoffset.

FIG. 15-(A) shows schematically simplified voltage waveforms of thescanning signal Vg(j) applied to the scanning signal line G(j) from thescanning signal line drive circuit 300 which is configured as the above.FIG. 15-(B) shows schematically simplified voltage waveforms of thescanning signal Vs(i) applied to the data signal line S(i) from the datasignal line drive circuit 200. FIG. 15-(C) shows schematicallysimplified voltage waveforms of the common electrode potential Vcs andthe opposed-electrode potential Vcom applied to the common electrodeline CS(j) and the opposed electrode Ec from the common electrode linedrive circuits CS and the opposed electrode drive circuit COMrespectively. FIG. 15-(D) shows schematically simplified voltagewaveform of the pixel potential Vd (i, j) in the pixel circuit P(i, j)which constitute the TFT substrate 100 according to the presentembodiment.

FIG. 16-(A) shows precise voltage waveforms in each pixel circuit P(i,j) (specifically, voltage waveforms on different locations on thescanning signal line G(j)) Vg(i, j) of a falling scanning signal Vg(j)outputted from the scanning signal line drive circuit 300 of theconfiguration described above. FIG. 16-(B) shows precise potentialwaveforms Vcs(i, j) in a part of common electrode line CS(j) whichoverlaps the pixel electrode 103 of the pixel circuit P(i, j), (morespecifically, potential waveforms when the scanning signal Vg(j) fallsfrom the gate-on voltage Vgh to the gate-off voltage Vgl). FIG. 16-(C)shows precise waveforms of the current which flows through the TFT 102of pixel circuit P(i, j) at the time when the scanning signal Vg(j)falls from the gate-on voltage Vgh to the gate-off voltage Vgl. FIG.16-(D) shows precise potential waveforms Vd(i, j) of the pixel electrode103 in the pixel circuit P(i, j) when the scanning signal Vg(j) fallsfrom the gate-on voltage Vgh to the gate-off voltage Vgl.

As shown in FIG. 10 and FIG. 11, a parasitic capacity is present betweenthe common electrode line CS(j) and the scanning signal line G(j), dueto the intervention by the capacity Cgd formed between the scanningsignal line and the pixel electrode and the capacity Ccs formed betweenthe pixel electrode and the common electrode line. For this reason, thepotential in the common electrode line CS(j) is affected by the voltagewaveform Vg(i, j) of the scanning signal in each pixel circuit P(i, j),and further, due to the signal propagation characteristic of the commonelectrode line CS(j), the potential waveform Vcs(i, j) in the commonelectrode line CS(j) changes as shown in FIG. 16-(B), depending on thelocation on the scanning signal line G(j).

Because of influences from the Vg(i, j) and the potential waveformVcs(i, j), as well as TFT characteristic differences (FIGS. 18-(A) and18-(B)), the waveform Id(i, j) of the current which flows through theTFT 102 during the time when the gate voltage in each TFT 102 is fallingfrom the gate-on voltage Vgh to the gate-off voltage Vgl takes differentforms as shown in FIG. 16-(C), depending on its location on the scanningsignal line G(j). Thus, the charge amount ΔQd(i, j) which transfers tothe pixel electrode 103 via the TFT 102 during the time when the gatevoltage in each TFT 102 falls from the gate-on voltage Vgh to thegate-off voltage Vgl takes different values depending on the location onthe scanning signal line G(j). Therefore, if all pixel circuits P(i, j)have the same parasitic capacity Cgd between the scanning signal lineand the pixel electrode (between the gate electrode and the drainelectrode of the TFT 102) as in the conventional TFT substrate 100, thepotential waveform Vd(i, j) of the pixel electrode 103 changes as shownin FIG. 16-(D), depending on the location on scanning signal line G(j),due to the differences in charge transfer to the pixel electrodes 103.As a result, even after a sufficient amount of time has passed since thescanning signal Vg(j) fell down to the gate-off voltage Vgl, the levelshift ΔVd(i, j) of the potential Vd(i, j) in each pixel electrode isdifferent depending on the location on the scanning signal line G(j),resulting in nonuniformity in the distribution of the level shift ΔVd,due to the differences of the charge amount ΔQd(i, j) based on theequation (5) discussed in the basic study. Specifically, the potentialVd(i, j) of the pixel electrode 103 changes as shown in FIG. 17-(A)depending on the location on the scanning signal line G(j). In morespecific words, the potential Vd(i, j) of the pixel electrode 103increases as the distance from the inputting end (scanning signal linedrive circuit 300) increases, attains a maximum value (peaks off) in acenter portion, and then decreases as the distance from the centerportion increases toward the terminating end. Near the terminating end,the value becomes substantially the same as of pixel potential Vd(1, j)near the inputting end. In response to these, the absolute value |ΔVd|of the level shift in the pixel potential Vd decreases as the distancefrom the inputting end increases, attains a minimum value near thecenter, and then increases as the distance from the center portionincreases toward the terminating end. Near the terminating end, thevalue becomes substantially the same as of |ΔVd| near the inputting end.

As described, according to the present embodiment, the pixel potentialVd(N, j) near the terminating end becomes substantially equal to thepixel potential Vd(1, j) near the inputting end, differing from thesecond embodiment in which the pixel potential Vd(N, j) near theterminating end does not decrease as low as the pixel potential Vd(1, j)near the inputting end (See FIG. 13-(A)). A reason for this is: in thepresent embodiment, voltage falling of the scanning signal Vg(j)outputted from the scanning signal line drive circuit 300 is controlled,so the falling edge gradient in the voltage waveform Vg(i, j) issubstantially the same at all locations on the scanning signal line G(j)(FIG. 16-(A)). This eliminates or reduces influences from the voltagewaveform Vg(i, j) in the scanning signal line G(j) to the nonuniformityof the level shift ΔVd, and the nonuniformity of the level shift ΔVd iscaused primarily by influences from the potential waveform Vcs(i, j) inthe common electrode line CS(j) (and based on the TFT characteristic).It should be appreciated that the potential in the common electrode lineCS(j) makes the biggest change at the center portion which is thelocation electrically farthest from the two common electrode line drivecircuits CS connected at two ends of the line (This center portion alsorepresents the center portion of the scanning signal line G(j)).Specifically, potential waveform Vcs(i, j) attains the highest waveheight at the center portion.

The present embodiment takes care of the above-described nonuniformityof the pixel potential Vd (i, j) or the level shift ΔVd, by forming eachpixel circuit P(i, j) so that the parasitic capacity Cgd between thescanning signal line and the pixel electrode (between the gate electrodeand the drain electrode of the TFT 102) in each pixel circuit P(i, j)will change as shown in FIG. 17-(B) in accordance with the location onthe scanning signal line G(j). Specifically, each pixel circuit P(i, j)is formed in such a way that the parasitic capacity Cgd or thecorrection amount ΔCgd will be substantially equal to |ΔQd/Vgpp|. Moreaccurately, values of the parasitic capacity Cgd are adjusted by meansof simulations for example so that (Vgpp·Cgd+ΔQd)/Cpix will be constant.This means that the parasitic capacity Cgd will increase as the distancefrom the inputting end of the scanning signal line G(j) increases,attains a maximum value (peaks off) in the center portion, and thendecreases as the distance from the center region increases toward theterminating end, and takes generally the same value as does near theinputting end, in the region near the terminating end. Thus, each pixelcircuit P(i, j) is formed so that its parasitic capacity Cgd will belarger as the circuit becomes electrically farther away from the commonelectrode line drive circuits CS. As a result, as shown in FIG. 17-(C),it becomes possible that in all pixel circuits P(i, j), the potential Vd(i, j) of the pixel electrode 103 and its level shift ΔVd havesubstantially the same values regardless of the location on the scanningsignal line G(j) (the location in the TFT substrate 100). It should beappreciated that the parasitic capacity Cgd can be changed in accordancewith its location on the scanning signal line G(j) by changing theoverlap area between the scanning signal line G(j) and the pixelelectrode 103 and/or the overlap area between the scanning signal lineG(j) and the drain electrode of the TFT 102. In more specific words, themethod disclosed in Patent Document 4 (Japanese Patent Laid-Open Hei11-84428 Gazette) can be utilized.

According to the present embodiment as described, nonuniformity of thelevel shift ΔVd is eliminated or reduced in a TFT substrate 100, whichserves as an active matrix substrate, where common electrode lines CS(j)are formed in parallel to scanning, signal lines G(j) and a commonelectrode potential Vcs is applied from two ends of each commonelectrode line CS(j), and a voltage waveform Vg(i, j) has asubstantially constant falling edge gradient at all locations on eachscanning signal line G(j) because of a trailing edge control over thescanning signal Vg(j) outputted from the scanning signal line drivecircuit 300. The nonuniformity is eliminated or reduced, due to thearrangement that the pixel circuits P(i, j) are so formed that theparasitic capacity Cgd will be different depending on the location onthe scanning signal line G(j) correspondingly to the distribution ofpixel potential Vd or of the level shift ΔVd. Thus, it is possible toprovide high-quality images with reduced flickers etc. in a liquidcrystal display device which uses a TFT substrate according to thepresent embodiment.

When the voltage falling in the scanning signal Vg(j) outputted from thescanning signal line drive circuit 300 is controlled and the fallingedge gradient in the voltage waveform Vg(i, j) is substantially the sameat all locations on the scanning signal line G(j) (FIG. 16-(A)),influences from the voltage waveform Vg(i, j) in the scanning signalline G(j) to the nonuniformity of the level shift ΔVd are eliminated orreduced, and the potential Vd (i, j) of the pixel electrode 103 changesas shown in FIG. 17-(A) depending on the location on the scanning signalline G(j). With this situation in mind, according to the presentembodiment, each pixel circuit P(i, j) is formed so that the parasiticcapacity Cgd between the scanning signal line and the pixel electrode ineach pixel circuit P(i, j) will change as shown in FIG. 17-(B) inaccordance with the location on the scanning signal line G(j), wherebyas shown in FIG. 17-(C), the potential Vd(i, j) of the pixel electrode103 and its level shift ΔVd have substantially constant values in allpixel circuits P(i, j) regardless of their location on the scanningsignal line G(j) (the location in the TFT substrate 100), i.e. makingpossible to uniformalize the distribution of level shift Vd. However,the falling edge gradient obtained by the above-described control overthe falling edge of the scanning signal Vg(j) may result in a situationwhere the potential Vd (i, j) of the pixel electrode 103 at eachlocation on the scanning signal line G(j) takes a value between the oneshown in FIG. 13-(A) and the one shown in FIG. 17-(A). In such a case,each pixel circuit P(i, j) is formed in such a way that the parasiticcapacity Cgd between the scanning signal line and the pixel electrodewill make an intermediary change between the change pattern in FIG.13-(B) and the change pattern in FIG. 17-(B) in each pixel circuit P(i,j) in accordance with the location on the scanning signal line G(j),whereby it is possible to make the potential Vd (i, j) of the pixelelectrode 103 and its level shift ΔVd have substantially constant valuesin all pixel circuits P(i, j) as shown in FIG. 13-(C) or FIG. 17-(C),regardless of their location on the scanning signal line G(j).

4. Variation

In each of the above-described embodiments, nonuniformity of the levelshift ΔVd is eliminated or reduced by forming the pixel circuits P(i, j)in such a way that the parasitic capacity Cgd will be different inaccordance with the location on the scanning signal line G(j) (morespecifically, the location in the TFT 100) correspondingly to thedistribution of pixel potential Vd or of the level shift ΔVd. However,the present invention is not limited to an arrangement in which theparasitic capacity Cgd is varied in accordance with the location.Alternatively to or together with this, the pixel circuits P(i, j) maybe formed so that the TFT characteristic will vary in accordance withthe location on the scanning signal line G(j) (location in the TFTsubstrate 100) correspondingly to the distribution of pixel potential Vdor of the level shift ΔVd, thereby eliminating or reducing thenonuniformity of the level shift ΔVd. In this case, in order to vary theTFT characteristic in accordance with the distribution of level shiftΔVd, formation of each pixel circuit P(i, j) may be made in such a waythat a ratio L/W of the channel length L and the channel width W of theTFT 102 will change in accordance with the location on the scanningsignal line G(j) (location in the TFT substrate 100). In more specificwords, in each of the above embodiment, each pixel circuit P(i, j) oreach TFT 102 may be formed in such a way that the ratio L/W will changein virtually the same way as the parasitic capacity Cgd does, inaccordance with the location on the scanning signal line G(j) (See FIG.7-(B), FIG. 13-(B) and FIG. 17-(B)). Specifically, each pixel circuitP(i, j) or each TFT 102 is formed in such a way that the ratio L/W inthe TFT 102 will increase as the electrical distance from the scanningsignal line drive circuit 300 increases, and as the electrical distancefrom the common electrode line drive circuit CS increases. Only one ofthe channel length L and the channel width W may be changed, or both maybe changed in combination. In addition, the perimeter length of thesource electrode or of the drain electrode, the area of contact betweenthe source electrode and the semiconductor layer, and the area ofcontact between the drain electrode and the semiconductor layer may alsobe varied in discretional combination.

Further, arrangements other than those described above also fall in thescope as long as the nonuniformity of the level shift ΔVd is eliminatedor reduced by varying an electrical characteristic value of an elementin the pixel circuit P(i, j), in accordance with the location on thescanning signal line G(j). For example, out of a plurality ofelectrostatic capacities which form the pixel capacity Cpix in eachpixel circuit P(i, j), at least one other than the parasitic capacityCgd may be varied in accordance with the location on the scanning signalline G(j). In this case, the varying can be accomplished, for example,by varying the common-electrode capacity (supplemental capacity) Ccs ineach pixel circuit P(i, j) in accordance with the location on thescanning signal line G(j). In this case, the common-electrode capacity(supplemental capacity) Ccs is decreased as the distance increases fromthe common electrode line drive circuits CS (the location where thecommon electrode potential Vcs is applied to the common electrode lineCS(j)). Also, in order that the equation (5) obtained in the basic studywill give a substantially constant value for all of the pixel circuitsin the TFT substrate (for elimination or reduction of the nonuniformityof level shift ΔVd), only one setting on a parameter such as the TFTcharacteristic or one of the electrostatic capacities (variouselectrostatic capacities formed between the pixel electrode and otherelectrodes) may be changed as was in each of the above-describedembodiments; however, a combination of settings on these parameters maybe used instead. It should be appreciated to note here that the pixelelectrode can mean any of the electrodes which are connected with thepixel electrode 103 while being separated from each signal line by theTFT and the insulation film. Therefore, if the elimination or reductionof the nonuniformity of level shift ΔVd is to be achieved by making asetting on the parasitic capacity Cgd (the electrostatic capacitybetween the TFT gate electrode and the drain electrode) for each pixelcircuit, the arrangement may be that the Cgd at one location in thepixel electrode is varied, or may be that Cgds′ at a plurality oflocations are varied in combination, or the configuration may be just asdescribed in the above, depending on the presence or absence of acertain Cgd(s). These pixel electrodes may not necessarily be of alow-resistance metal such as Al (aluminum) but may include ahigh-resistance film such as a semiconductor layer.

According to the embodiments, each pixel circuit P(i, j) is formed insuch a way that the parasitic capacity Cgd, L/W of the TFT or otherswill change smoothly in accordance with the distribution of the pixelpotential Vd or the level shift ΔVd, to eliminate or reduce thenonuniformity of the level shift ΔVd (See FIG. 7-(B), FIG. 13-(B) andFIG. 17-(B)); however, the present invention is not limited to this. Thechange in the parasitic capacity Cgd, L/W of the TFT, and others may beof a stepped pattern, a polygonal line pattern, a nesting pattern or amosaic pattern, or a combination of these as long as the pattern followsthe distribution of the level shift ΔVd. In the light of improveddisplay quality however, an arrangement for smooth change of theparasitic capacity Cgd, TFT L/W, or others is preferable.

According to the second and the third embodiments, the common electrodeline is in parallel to the scanning signal line; however, thearrangement may be whatever else as long as a predeterminedelectrostatic capacity (an equivalent of the common-electrode capacityor the supplemental capacity) is formed between the line and the pixelelectrode. Further, the common electrode line may ride on a plurality ofscanning signal lines, or a plurality of data signal lines. A pluralityof the common electrode lines may be provided per a pixel circuit or pera pixel electrode, or the common electrode line may be provided as aplate. As exemplified, if the common electrode line has a differentconfiguration from those in the second and the third embodiments, thesame advantages can be achieved by e.g. forming each pixel circuit insuch a way that the parasitic capacity (the electrostatic capacitybetween the gate electrode and the drain electrode in the TFT) Cgd orthe ratio L/W between the channel length L and the channel width W willincrease as the distance from the common electrode line drive circuitincreases, so that the level shift nonuniformity of the pixel potentialis eliminated or reduced. It should be appreciated that the presentinvention is applicable also to a case in Patent Document 2 (JapanesePatent Laid-Open No. 2001-33758 Gazette) where common electrode linesare divided into a plurality of groups, or to a case such as inline-inversion drive method where the potential in the common electrodeline is not constant but fluctuates.

According to each of the embodiments, influence of signal propagationdelay in the opposed electrode is assumed to be small enough to benegligible. However, depending on the resistance value and/or the shapeof the opposed electrode, influence of signal propagation delay in theopposed electrode cannot be negligible. However, even in such a case,the level shift nonuniformity of the pixel potential can be eliminatedor reduced by changing the parasitic capacity Cgd, the ratio L/W of theTFT channel length L and the channel width W or others in accordance tothe location, as exemplified in the second and the third embodiments byarrangements to deal with the influences from the signal propagationdelay characteristic in the common electrode line.

According to each of the embodiments, the opposed electrode is providedon the opposed substrate which is a different substrate from the TFTsubstrate, being the other of a pair of substrates that sandwich theliquid crystal, and the liquid crystal is driven by a vertical electricfield which is perpendicular to the substrate. There are cases, however,in which the opposed electrode is formed on the same substrate as is thepixel electrode (a case where the opposed electrode is formed on the TFTsubstrate) or in which the common electrode serves also as the opposedelectrode. Even in such cases as the above, where the liquid crystal isdriven by a horizontal electric field which is in parallel to thesubstrate, the present invention is applicable as means for eliminatingor reducing level shift in the pixel potential.

In each of the embodiments, description was made by taking a TFTsubstrate as an example of an active matrix substrate used in a liquidcrystal display device. However, the present invention is applicable toother display devices than liquid crystal display devices as long assuch a device uses an active matrix substrate provided with a matrix ofpixel circuits each including an electrostatic capacity having virtuallythe same voltage holding capability as the above-described pixelcapacity constituted by a pixel electrode and other electrodes, and athin-film transistor; as well as with scanning signal lines and datasignal lines disposed in a grid pattern. For example, the presentinvention may be applied to an active matrix substrate used in anorganic EL (Electro luminescence) display device as means foreliminating or reducing level shift in the holding voltage in thecapacity which is equivalent to the pixel capacity which has beendiscussed so far. In this case, a capacitor which has theabove-mentioned electrostatic capacity for holding a voltage thatrepresents a pixel value is provided by: a voltage holding electrodewhich is connected with the drain electrode of the TFT in the pixelcircuit (This voltage holding electrode serves as an equivalent to thepixel electrode thus far discussed); and an electrode of a power sourceline or of a grounding line which corresponds to the common electrodeline thus far discussed. Note, however, that depending on the drivingmethod of the organic EL display device, a second TFT serving as aswitching element is placed between the source electrode of the firstTFT and the data signal line. Still in another case, a second TFTserving as a switching element and a capacity element (capacitor)connected in series thereto are placed between the source electrode ofthe first TFT and the data signal line.

For a pixel circuit to be used in an organic EL display device, aconfiguration as shown in FIG. 20 can be used (See Japanese PatentLaid-Open No. 2001-147659 Gazette). In this pixel circuit, selection ofscanning lines scanA and scanB turns ON a TFT 3 and a TFT 4, allowing acurrent from a current source CS to flow to a TFT 1, and a gate-sourcevoltage which corresponds to the current flowing to the TFT 1 is chargedin a holding capacitor C. Thereafter, when the scanning line scan Bassumes a non-selected state, a TFT 4 is turned OFF to hold the voltagecharged in the holding capacitor C. As for a drive TFT 2, a current inaccordance with the charge voltage at the holding capacitor C flowsthrough the drive TFT 2, and this current causes a light emittingelement OLED to illuminate. When the TFT 4 changes its state from ON toOFF in this operation, a level shift is caused, just like in theembodiments described earlier, by a parasitic capacity Cpa of the TFT 4.If the level shift varies among the pixel circuits, emission luminancevaries and display quality decreases. In such a pixel circuit as this, aportion indicated with a symbol “A” serves as a voltage holdingelectrode which constitutes a voltage holding capacitor C. A data line“data” is connected with this voltage holding electrode (A) via the TFT3 and the TFT 4 which serve as switching elements. The TFT 4 is turnedON/OFF by the scanning line scan B. The parasitic capacity Cpa in theTFT 4 corresponds to the parasitic capacity Cgd in the first and thesecond embodiments which was present in a TFT 102 in the pixel circuit.Therefore, the present invention is also applicable to an active matrixsubstrate used in organic EL display devices which include pixelcircuits of a configuration shown in FIG. 20 to accomplishuniformalization of the level shift within the substrate.

Other organic EL display devices use a different configuration in theirpixel circuits, such as one exemplified in FIG. 21 (See Japanese PatentLaid-Open No. 2002-156923 Gazette). In this pixel circuit, selection ofa scanning line 25 (scan) turns ON a TFT 24, causing a data voltage in adata line 26 “data” to be held in a holding capacity 23(Cs). Thereafter,when the scanning line 25 assumes a non-selected state, the TFT 24 isturned OFF, the data voltage held in the holding capacity 23 ismaintained, and a current in accordance with the voltage flows throughthe drive TFT 22, and this current causes an organic EL device 21 toilluminate. However, when the TFT 24 changes its state from ON to OFF, alevel shift is caused like in the embodiments described above, due to aparasitic capacity Cg2 in the TFT 4. If the level shift varies among thepixel circuits, emission luminance varies and display quality decreases.In such a pixel circuit as this, a portion indicated with a symbol “A”serves as a voltage holding electrode which constitutes a voltageholding capacity 23. The data line 26 is connected with this voltageholding electrode (A) via the TFT 24. The TFT 24 is turned ON/OFF by thescanning line 25. The parasitic capacity Cgs2 in the TFT 24 correspondsto the parasitic capacity Cgd in the first and the second embodimentswhich was present in a TFT 102 in the pixel circuit. Therefore, thepresent invention is also applicable to an active matrix substrate usedin organic EL display devices which include pixel circuits of theconfiguration shown in FIG. 21, to accomplish uniformalization of thelevel shift within the substrate.

As in each embodiment described above, an active matrix substrate usedin a liquid crystal display device is driven in AC. However, the presentinvention is also applicable to cases where the drive is performed inDC, such as in an active matrix substrate used in organic EL displaydevices.

It should be appreciated that according to each of the embodiments, thedrive circuit (such as the data signal line drive circuit 200 and thescanning signal line drive circuit 300) for driving the TFT substrate100 as an active matrix substrate is provided as a separate componentmanufactured independently from the TFT substrate 100. Alternatively,the drive circuit may be formed on the TFT substrate 100 (The activematrix substrate may be of a driver-monolithic type). As a further note,the pixel capacity Cpix in each pixel circuit in the embodiments isexpressed as Cpix=Cgd+Ccs+Clc. However, in case where other parasiticcapacities are not negligible, pixel capacity Cpix may include thoseparasitic capacities. Further, if an active matrix substrate accordingto the present invention is formed with common electrode lines, thepotential Vcs in the common electrode line and the potential Vcom in theopposed electrode may not necessarily be equal to each other. Stillfurther, in the second and the third embodiments, the common electrodeline is formed individually from the scanning signal line;alternatively, the common electrode line in each pixel circuit may alsoserve as the scanning signal line for an adjacent pixel circuit.

INDUSTRIAL APPLICABILITY

The present invention is for application to an active matrix substrateor a drive circuit therefor used in display devices, sensors, etc., andparticularly suitable to active matrix substrates for liquid crystaldisplay devices and EL display devices.

The invention claimed is:
 1. An active matrix substrate comprising: datasignal lines each for one of data signals; scanning signal linescrossing with the data signal lines; a matrix of pixel circuits eachcorresponding to one of intersections made by the data signal lines andthe scanning signal lines; wherein each pixel circuit includes: afield-effect transistor having: a source electrode connected with one ofthe data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; and a gate electrode connected with one of thescanning signal lines that passes through the correspondingintersection; and a voltage holding electrode connected with a drainelectrode of the field-effect transistor, providing a predeterminedvoltage holding capacitor; and wherein an electrostatic capacity Cgdbetween the gate electrode and the drain electrode in the field-effecttransistor increases whereas a rate of the increase of the electrostaticcapacity Cgd decreases with an increasing electrical distance from alocation of signal application for driving the scanning signal linewhich passes through the corresponding intersection.
 2. A drive circuitfor the active matrix substrate according to claim 1, comprising: ascanning signal line drive circuit for selectively driving the scanningsignal by applying predetermined scanning signals respectively to thescanning signal lines, wherein the scanning signal line drive circuitcontrols a speed of electric potential change when the scanning signalsmake a transition from a predetermined ON voltage which turns thefield-effect transistors into a conductive state to a predetermined OFFvoltage which turns the field-effect transistors into a nonconductivestate.
 3. The drive circuit according to claim 2, wherein the scanningsignal line drive circuit controls the speed of electric potentialchange of the scanning signals to be outputted from the scanning signalline drive circuit, based on a signal propagation delay characteristicof the scanning signal lines, so that the speed of electric potentialchange will be substantially equal regardless of the location on thescanning signal lines.
 4. A display device comprising the active matrixsubstrate according to claim 1 and a drive circuit for driving theactive matrix substrate.
 5. An active matrix substrate comprising: datasignal lines each for one of data signals; scanning signal linescrossing with the data signal lines; and a matrix of pixel circuits eachcorresponding to one of intersections made by the data signal lines andthe scanning signal lines; wherein each pixel circuit includes: afield-effect transistor having: a source electrode connected with one ofthe data signal lines that passes through a corresponding one of theintersections, directly or via a predetermined switching element and/ora capacity element; a gate electrode connected with one of the scanningsignal lines that passes through the corresponding intersection; and avoltage holding electrode connected with a drain electrode of thefield-effect transistor, providing a predetermined voltage holdingcapacitor; and wherein an area of overlap between the electrodeconstituting the scanning signal line that passes through thecorresponding intersection and the voltage holding electrode or thedrain electrode of the field-effect transistor increases whereas a rateof the increase of the area decreases with an increasing electricaldistance from a location of signal application for driving the scanningsignal line which passes through the corresponding intersection.
 6. Adrive circuit for the active matrix substrate according to claim 5,comprising: a scanning signal line drive circuit for selectively drivingthe scanning signal by applying predetermined scanning signalsrespectively to the scanning signal lines, wherein the scanning signalline drive circuit controls a speed of electric potential change whenthe scanning signals make a transition from a predetermined ON voltagewhich turns the field-effect transistors into a conductive state to apredetermined OFF voltage which turns the field-effect transistors intoa nonconductive state.
 7. The drive circuit according to claim 6,wherein the scanning signal line drive circuit controls the speed ofelectric potential change of the scanning signals to be outputted fromthe scanning signal line drive circuit, based on a signal propagationdelay characteristic of the scanning signal lines, so that the speed ofelectric potential change will be substantially equal regardless of thelocation on the scanning signal lines.
 8. A display device comprisingthe active matrix substrate according to claim 5 and a drive circuit fordriving the active matrix substrate.
 9. An active matrix substratecomprising: data signal lines each for one of data signals; scanningsignal lines crossing with the data signal lines; a matrix of pixelcircuits each corresponding to one of intersections made by the datasignal lines and the scanning signal lines; wherein each pixel circuitincludes: a field-effect transistor having: a source electrode connectedwith one of the data signal lines that passes through a correspondingone of the intersections, directly or via a predetermined switchingelement and/or a capacity element; a gate electrode connected with oneof the scanning signal lines that passes through the correspondingintersection; and a voltage holding electrode connected with a drainelectrode of the field-effect transistor, providing a predeterminedvoltage holding capacitor; and wherein a ratio L/W between a channellength L and a channel width W in the field-effect transistor increaseswhereas a rate of the increase in the ratio L/W decreases with anincreasing electrical distance from a location of signal application fordriving the scanning signal line which passes through the correspondingintersection.
 10. A drive circuit for the active matrix substrateaccording to claim 9, comprising: a scanning signal line drive circuitfor selectively driving the scanning signal by applying predeterminedscanning signals respectively to the scanning signal lines, wherein thescanning signal line drive circuit controls a speed of electricpotential change when the scanning signals make a transition from apredetermined ON voltage which turns the field-effect transistors into aconductive state to a predetermined OFF voltage which turns thefield-effect transistors into a nonconductive state.
 11. The drivecircuit according to claim 10, wherein the scanning signal line drivecircuit controls the speed of electric potential change of the scanningsignals to be outputted from the scanning signal line drive circuit,based on a signal propagation delay characteristic of the scanningsignal lines, so that the speed of electric potential change will besubstantially equal regardless of the location on the scanning signallines.
 12. A display device comprising the active matrix substrateaccording to claim 9 and a drive circuit for driving the active matrixsubstrate.
 13. An active matrix substrate comprising: data signal lineseach for one of data signals; scanning signal lines crossing with thedata signal lines; a matrix of pixel circuits each corresponding to oneof intersections made by the data signal lines and the scanning signallines; wherein each pixel circuit includes: a field-effect transistorhaving: a source electrode connected with one of the data signal linesthat passes through a corresponding one of the intersections, directlyor via a predetermined switching element and/or a capacity element; agate electrode connected with one of the scanning signal lines thatpasses through the corresponding intersection; and a voltage holdingelectrode connected with a drain electrode of the field-effecttransistor, providing a predetermined voltage holding capacitor; andwherein at least one of the electrostatic capacities which are formedbetween the drain electrode of the field-effect transistor or thevoltage holding electrode and other electrodes other than theelectrostatic capacity Cgd between the gate electrode and the drainelectrode of the field-effect transistor decreases while a rate of thedecrease of said at least one electrostatic capacity decreases with anincreasing electrical distance from a location of signal application fordriving the scanning signal line which passes through the correspondingintersection.
 14. A drive circuit for the active matrix substrateaccording to claim 13, comprising: a scanning signal line drive circuitfor selectively driving the scanning signal by applying predeterminedscanning signals respectively to the scanning signal lines, wherein thescanning signal line drive circuit controls a speed of electricpotential change when the scanning signals make a transition from apredetermined ON voltage which turns the field-effect transistors into aconductive state to a predetermined OFF voltage which turns thefield-effect transistors into a nonconductive state.
 15. The drivecircuit according to claim 14, wherein the scanning signal line drivecircuit controls the speed of electric potential change of the scanningsignals to be outputted from the scanning signal line drive circuit,based on a signal propagation delay characteristic of the scanningsignal lines, so that the speed of electric potential change will besubstantially equal regardless of the location on the scanning signallines.
 16. A display device comprising the active matrix substrateaccording to claim 13 and a drive circuit for driving the active matrixsubstrate.